intel/fs: Try to vectorize header setup in lower_load_payload().

intel/fs: Try to vectorize header setup in lower_load_payload().

In cases where LOAD_PAYLOAD is provided a pair of contiguous registers
as header sources, try to use a single SIMD16 instruction in order to
initialize them.  This is unlikely to affect the overall cycle count
of the shader, since the compressed instruction has twice the issue
time, except due to the reduced pressure on the instruction cache.

Main motivation is avoiding instruction-count regressions in
combination with the following copy propagation improvements, which
will allow the SIMD16 g0-1 header setup emitted for framebuffer writes
to be copy-propagated into its LOAD_PAYLOAD, leading to the emission
of two SIMD8 MOV instructions instead of a single SIMD16 MOV.

Reverting this commit on top of the copy propagation changes would
lead to the following shader-db regressions on SKL and other
platforms:

 total instructions in shared programs: 14926738 -> 14935415 (0.06%)
 instructions in affected programs: 1892445 -> 1901122 (0.46%)
 helped: 0
 HURT: 8676

Without the following copy propagation changes this doesn't have any
effect on shader-db on Gen7+, because we would typically set up the FB
write header with a separate SIMD16 MOV that isn't currently
copy-propagated into the LOAD_PAYLOAD, so the individual SIMD8 MOVs
result of LOAD_PAYLOAD lowering would get register-coalesced away
under normal circumstances.  However that wasn't the case for MRF
LOAD_PAYLOAD destinations on Gen6 and earlier, because register
coalesce only kicks in for GRFs, leaving a number of redundant SIMD8
MOVs lying around.  On SNB this leads to the following shader-db
improvements:

 total instructions in shared programs: 10770538 -> 10734681 (-0.33%)
 instructions in affected programs: 2700655 -> 2664798 (-1.33%)
 helped: 17791
 HURT: 0

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