radeon/llvm: Run LLVM's instruction combining pass

radeon/llvm: Run LLVM's instruction combining pass

This should improve code quality in general and will help with some
future changes to how we emit kill instructions.

shader-db shows a few regressions, but these don't seem to be the result
of deficiencies in instcombine.  They're mostly caused by the scheduler
making different decisions than before.

shader-db stats (bonaire):

979 shaders
Totals:
SGPRS: 35056 -> 34872 (-0.52 %)
VGPRS: 20624 -> 20696 (0.35 %)
Code Size: 764372 -> 749032 (-2.01 %) bytes
LDS: 11 -> 11 (0.00 %) blocks
Scratch: 12288 -> 12288 (0.00 %) bytes per wave

Totals from affected shaders:
SGPRS: 13264 -> 13072 (-1.45 %)
VGPRS: 8248 -> 8316 (0.82 %)
Code Size: 486320 -> 470992 (-3.15 %) bytes
LDS: 11 -> 11 (0.00 %) blocks
Scratch: 11264 -> 11264 (0.00 %) bytes per wave

Increases:
SGPRS: 6 (0.01 %)
VGPRS: 20 (0.02 %)
Code Size: 14 (0.01 %)
LDS: 0 (0.00 %)
Scratch: 0 (0.00 %)

Decreases:
SGPRS: 32 (0.03 %)
VGPRS: 8 (0.01 %)
Code Size: 244 (0.25 %)
LDS: 0 (0.00 %)
Scratch: 0 (0.00 %)

*** BY PERCENTAGE ***

Max Increase:

SGPRS: 32 -> 48 (50.00 %)
VGPRS: 12 -> 20 (66.67 %)
Code Size: 216 -> 224 (3.70 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Scratch: 0 -> 0 (0.00 %) bytes per wave

Max Decrease:

SGPRS: 40 -> 32 (-20.00 %)
VGPRS: 16 -> 12 (-25.00 %)
Code Size: 368 -> 280 (-23.91 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Scratch: 0 -> 0 (0.00 %) bytes per wave

*** BY UNIT ***

Max Increase:

SGPRS: 32 -> 48 (50.00 %)
VGPRS: 28 -> 36 (28.57 %)
Code Size: 39320 -> 40132 (2.07 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Scratch: 0 -> 0 (0.00 %) bytes per wave

Max Decrease:

SGPRS: 72 -> 64 (-11.11 %)
VGPRS: 48 -> 40 (-16.67 %)
Code Size: 6272 -> 5852 (-6.70 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Scratch: 0 -> 0 (0.00 %) bytes per wave

Reviewed-by: Marek Olšák <marek.olsak@amd.com>