i965/blorp: Emit sample index in SAMPLE_LD message when necessary

This patch modifies the function brw_blorp_blit_program::texel_fetch()
to emit the SI (sample index) argument to the SAMPLE_LD message when
reading from a sample index other than zero.

Previously we were using the ugly hack of configuring multisampled
source surfaces as single-sampled, and accessing sample indices other
than zero by swizzling the texture coordinates in the WM program.
i965/blorp: Use MSDISPMODE_PERSAMPLE rendering when necessary

This patch modifies the "blorp" WM program so that it can be run in
MSDISPMODE_PERSAMPLE (which means that every single sample of a
multisampled render target is dispatched to the WM program, not just
every pixel).

Previously we were using the ugly hack of configuring multisampled
destination surfaces as single-sampled, and generating sample indices
other than zero by swizzling the pixel coordinates in the WM program.
i965/blorp: Factor gen6_blorp_emit_batch_head into separate functions.

This patch separates out the portions of gen6_blorp_emit_batch_head()
that emit 3DSTATE_MULTISAMPLE, 3DSTATE_SAMPLE_MASK, and
STATE_BASE_ADDRESS.  This paves the way for making the blorp code work
on Gen7, where additional command packets
(3DSTATE_PUSH_CONSTANT_ALLOC_VS and 3DSTATE_PUSH_CONSTANT_ALLOC_PS)
need to be emitted before 3DSTATE_MULTISAMPLE.
i965/blorp: Allocate space for push constants on Gen7.

On Gen7, push constants for shader programs are stored in the URB, so
blorp code needs to set aside space for them.  This was previously
unnecessary because blorp code was based on HiZ operations, which
don't require any shaders.

This patch adds a call from gen7_blorp_exec() to
gen7_allocate_push_constants(), to ensure that push constants are
assigned the correct location in the URB.  It also extracts a new
function gen7_emit_urb_state() from gen7_upload_urb(), which is
re-used by gen7_blorp_emit_urb_config() to ensure that the URB regions
used by all the pipeline stages leave room for the push constants.
i965/blorp: Use 16 pixel dispatch on Gen7 when there is a WM program.

When executing a blorp operation on Gen7 that does not need a WM
program (i.e. a HiZ operation), we use 32-pixel dispatch mode because
it's faster.  However, when executing a blorp operation that does need
a WM program, we need to use 16-pixel dispatch, because blorp WM
programs are compiled assuming 16-pixel dispatch.
i965/blorp: Implement proper texel fetch messages for Gen7.

On Gen6, texel fetch is always accomplished using the SAMPLE_LD
message, which accepts arguments (u, v, r, lod, si).  On Gen7, there
are two* texel fetch messages: SAMPLE_LD for non-MSAA surfaces, taking
arguments (u, lod, v), and SAMPLE_LD2DSS for MSAA surfaces, taking
arguments (si, u, v).

*Technically, there are other texel fetch messages, but they are used
for "compressed" MSAA surfaces, which we don't yet support.

This patch adds the proper message types and argument orderings for
Gen7.
i965/blorp: Enable blorp blits on Gen7.

Gen7 support for blorp (blits using the render bath) now works for
non-MSAA purposes.  This patch enables it.

Since blorp operations re-use the logic for HiZ ops, this required
adding a case to the switch statement in gen7_blorp_emit_wm_config(),
to allow for the case where no HiZ op is being performed.
i965/msaa: Add defines for Gen7.
i965/msaa: Properly handle sliced layout for Gen7.

Starting in Gen7, there are two possible layouts for MSAA surfaces:

- Interleaved, in which additional samples are accommodated by scaling
  up the width and height of the surface.  This is the only layout
  available in Gen6.  On Gen7 it is used for depth and stencil
  surfaces only.

- Sliced, in which the surface is stored as a 2D array, with array
  slice n containing all pixel data for sample n.  On Gen7 this layout
  is used for color surfaces.

The "Sliced" layout has an additional requirement: it must be used in
ARYSPC_LOD0 mode, which means that the surface doesn't leave any extra
room between array slices for miplevels other than 0.

This patch modifies the surface allocation functions to use the
correct layout when allocating MSAA surfaces in Gen7, and to set the
array offsets properly when using ARYSPC_LOD0 mode.  It also modifies
the code that populates SURFACE_STATE structures to ensure that
ARYSPC_LOD0 mode is selected in the appropriate circumstances.
i965/msaa: Validate Gen7 surface state constraints.

When a Gen7 SURFACE_STATE is configured for MSAA, a number of
additional constaints come in to play.  This patch adds a function
gen7_check_surface_setup() which verifies that all of those
constraints are met.
i965/msaa: Modify blorp code to account for Gen7 MSAA layouts.

Since blorp uses color textures and render targets to do all its work
(even when blitting stencil and depth data), it always has to
configure the Gen7 GPU to use the new "sliced" MSAA layout.  However,
when blitting stencil or depth data, the actual MSAA layout is
interleaved (as in Gen6).  Therefore, blorp has to do extra coordinate
transformation work to account for the interleaving manually.

This patch causes blorp to perform the necessary extra coordinate
transformations.

It also modifies the blorp SURFACE_STATE setup code for Gen7, so that
it does not try to correct the surface width and height to account for
MSAA, since "sliced" MSAA layout doesn't affect the surface width or
height.
i965/msaa: Implement manual blending operation for Gen7.

On Gen6, the blending necessary to blit an MSAA surface to a non-MSAA
surface could be accomplished with a single texturing operation.  On
Gen7, the WM program must fetch each sample and blend them together
manually. This patch implements the manual blend operation.
i965/msaa: Enable 4x MSAA on Gen7.

Basic 4x MSAA support now works on Gen7.  This patch enables it.

As with Gen6, MSAA support is still fairly preliminary.  In
particular, the following are not yet supported:
- 8x oversampling (Gen7 has hardware support for this, but we do not
  yet expose it).
- Fully general blits between MSAA and non-MSAA buffers.
- Formats other than RGBA8, DEPTH24, and STENCIL8.
- Centrold interpolation.
- Coverage parameters (glSampleCoverage, GL_SAMPLE_ALPHA_TO_COVERAGE,
  GL_SAMPLE_ALPHA_TO_ONE, GL_SAMPLE_COVERAGE, GL_SAMPLE_COVERAGE_VALUE,
  GL_SAMPLE_COVERAGE_INVERT).
i965/blorp: Implement destination clipping and scissoring

This patch implements clipping and scissoring of the destination rect
for blits that use the blorp engine (e.g. MSAA blits).
glsl/constant propagation: kill whole var if LHS involves array indexing.

When considering which components of a variable were killed by an
assignment, constant propagation would previously just use the write
mask of the assignment.  This worked if the LHS of the assignment was
simple, e.g.:

v.xy = ...; // (assign (xy) (var_ref v) ...)

But it did the wrong thing if the LHS of the assignment involved an
array indexing operator, since in this case the write mask is always
(x):

v[i] = ...; // (assign (x) (deref_array (var_ref v) (var_ref i)) ...)

In general, we can't predict which vector component will be selected
by array indexing, so the only safe thing to do in this case is to
kill the entire variable.

Fixes piglit tests {fs,vs}-vector-indexing-kills-all-channels.shader_test.
WIP: get RGB MSAA to work
MESA_FORMAT_A8 works now
MESA_FORMAT_RGB565 works now
MESA_FORMAT_RGBA_16 works now
MESA_FORMAT_ARGB4444 works now.