!UPSTREAM: i965/vec4: Add the ability for attributes to be interleaved.

When geometry shaders are operated in "single" or "dual instanced"
mode, a single set of geometry shader inputs is interleaved into the
thread payload (with each payload register containing a pair of
inputs) in order to save register space.

This patch modifies vec4_visitor::lower_attributes_to_hw_regs so that
it can handle the interleaved format.

TODO:
- test.
!UPSTREAM: i965/gs: Add the ability to compile a DUAL_INSTANCED geometry shader.

Not yet used.

TODO:
- test.
!UPSTREAM: i965/vec4: Add the ability to suppress register spilling.

In future patches, this will allow us to first try compiling a
geometry shader in DUAL_OBJECT mode (which is more efficient but uses
more registers) and then if spilling is required, fall back on
DUAL_INSTANCED mode.

TODO:
- Test.
!UPSTREAM: i965/gs: If a DUAL_OBJECT gs would spill, fall back to DUAL_INSTANCED.

This is similar to what we do for 16-wide vs 8-wide fragment shaders.
First we try compiling the geometry shader in DUAL_OBJECT mode.  If we
can't do that without spilling, we fall back on DUAL_INSTANCED mode,
which should require less spilling (since it uses an interleaved
layout of payload registers).

In an ideal world we'd fall back to SINGLE mode, which would allow us
to interleave general-purpose registers too (resulting in even less
likelihood of spilling).  But at the moment, the vec4 generator and
visitor classes don't have the infrastructure to interleave general
purpose registers, so DUAL_INSTANCED is the best we can do.

As a side benefit this paves the way for implementing instanced
geometry shaders (which are incompatible with DUAL_OBJECT mode).

TODO: test.