radeon/llvm: Improve codegen for KILL_IF

radeon/llvm: Improve codegen for KILL_IF

Rather than emitting one kill instruction per component of KILL_IF's src
reg, we now or the components of the src register together and use the
result as a condition for just one kill instruction.

shader-db stats (bonaire):

979 shaders
Totals:
SGPRS: 34872 -> 34848 (-0.07 %)
VGPRS: 20696 -> 20676 (-0.10 %)
Code Size: 749032 -> 748452 (-0.08 %) bytes
LDS: 11 -> 11 (0.00 %) blocks
Scratch: 12288 -> 12288 (0.00 %) bytes per wave

Totals from affected shaders:
SGPRS: 1184 -> 1160 (-2.03 %)
VGPRS: 600 -> 580 (-3.33 %)
Code Size: 13200 -> 12620 (-4.39 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Scratch: 0 -> 0 (0.00 %) bytes per wave

Increases:
SGPRS: 2 (0.00 %)
VGPRS: 0 (0.00 %)
Code Size: 0 (0.00 %)
LDS: 0 (0.00 %)
Scratch: 0 (0.00 %)

Decreases:
SGPRS: 5 (0.01 %)
VGPRS: 5 (0.01 %)
Code Size: 25 (0.03 %)
LDS: 0 (0.00 %)
Scratch: 0 (0.00 %)

*** BY PERCENTAGE ***

Max Increase:

SGPRS: 32 -> 40 (25.00 %)
VGPRS: 0 -> 0 (0.00 %)
Code Size: 0 -> 0 (0.00 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Scratch: 0 -> 0 (0.00 %) bytes per wave

Max Decrease:

SGPRS: 32 -> 24 (-25.00 %)
VGPRS: 16 -> 12 (-25.00 %)
Code Size: 116 -> 96 (-17.24 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Scratch: 0 -> 0 (0.00 %) bytes per wave

*** BY UNIT ***

Max Increase:

SGPRS: 64 -> 72 (12.50 %)
VGPRS: 0 -> 0 (0.00 %)
Code Size: 0 -> 0 (0.00 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Scratch: 0 -> 0 (0.00 %) bytes per wave

Max Decrease:

SGPRS: 32 -> 24 (-25.00 %)
VGPRS: 16 -> 12 (-25.00 %)
Code Size: 424 -> 356 (-16.04 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Scratch: 0 -> 0 (0.00 %) bytes per wave

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