i965/gs: Fix up gl_PointSize input swizzling for DUAL_INSTANCED gs.

Geometry shaders that run in "DUAL_INSTANCED" mode store their inputs
in vec4's.  This means that when compiling gl_PointSize input
swizzling (a MOV instruction which uses a geometry shader input as
both source and destination), we need to do two things:

- Set force_writemask_all to ensure that the MOV happens regardless of
  which channels are enabled.

- Set the source register region to <4;4,1> (instead of <0;4,1> to
  satisfy register region restrictions.
!UPSTREAM: Fixes to vertex reordering workaround

TODO:
- Fold into vertex reordering code.
!UPSTREAM: i965/gs: fix up primitive ID workaround for DUAL_INSTANCE dispatch.

TODO:

- Add better explanatory comments in generate_vec4_instruction.
!UPSTREAM: i965: Reduce gl_MaxGeometryInputComponents to 64.

Although in principle there is no hardware limitation that prevents
gl_MaxGeometryInputComponents from being set to 128 on Gen7, we have
the following limitations in the vec4 compiler back end:

- Registers assigned to geometry shader inputs can't be spilled or
  later re-used for any other purpose.

- The last 16 registers are set aside for the "MRF hack", meaning they
  can only be used to send messages, and not for general purpose
  computation.

- Up to 32 registers may be reserved for push constants, even if there
  is sufficient register pressure to make this impractical.

A shader using 128 geometry input components, and having an input type
of triangles_adjacency, would use up:

- 1 register for r0 (which holds URB handles and various pieces of
  control information).

- 1 register for gl_PrimitiveID.

- 102 registers for geometry shader inputs (17 registers per input
  vertex, assuming DUAL_INSTANCED dispatch mode and allowing for one
  register of overhead for gl_Position and gl_PointSize, which are
  present in the URB map even if they are not used).

- Up to 32 registers for push constants.

- 16 registers for the "MRF hack".

That's a total of 152 registers, which is well over the 128 registers
the hardware supports.

Fortunately, the GLSL 1.50 spec allows us to reduce
gl_MaxGeometryInputComponents to 64.  Doing that frees up 48
registers, brining the total down to 104 registers, leaving 24
registers available to do computation.