intel/fs: Add partial support for copy-propagating FIXED_GRFs.

intel/fs: Add partial support for copy-propagating FIXED_GRFs.

This will be useful for eliminating redundant copies from the FS
thread payload, particularly in SIMD32 programs.  For the moment we
only allow FIXED_GRFs with identity strides in order to avoid dealing
with composing the arbitrary bidimensional strides that FIXED_GRF
regions potentially have, which are rarely used at the IR level
anyway.

This enables the following commit allowing block-propagation of
FIXED_GRF LOAD_PAYLOAD copies, and prevents the following shader-db
regressions (including SIMD32 programs) in combination with the
interpolation rework part of this series.  On ICL:

   total instructions in shared programs: 20484665 -> 20529650 (0.22%)
   instructions in affected programs: 6031235 -> 6076220 (0.75%)
   helped: 5
   HURT: 42073

   total spills in shared programs: 8748 -> 8925 (2.02%)
   spills in affected programs: 186 -> 363 (95.16%)
   helped: 5
   HURT: 9

   total fills in shared programs: 8663 -> 8960 (3.43%)
   fills in affected programs: 647 -> 944 (45.90%)
   helped: 5
   HURT: 9

On SKL:

   total instructions in shared programs: 18937442 -> 19128162 (1.01%)
   instructions in affected programs: 8378187 -> 8568907 (2.28%)
   helped: 39
   HURT: 68176

   LOST:   1
   GAINED: 4

On SNB:

   total instructions in shared programs: 14094685 -> 14243499 (1.06%)
   instructions in affected programs: 7751062 -> 7899876 (1.92%)
   helped: 623
   HURT: 53586

   LOST:   7
   GAINED: 25

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>