nir: propagate known constant values into the if-then branch

nir: propagate known constant values into the if-then branch

Helps Max Waves / VGPR use in a bunch of Unigine Heaven
shaders.

shader-db results radeonsi (VEGA):
Totals from affected shaders:
SGPRS: 5505440 -> 5505872 (0.01 %)
VGPRS: 3077520 -> 3077296 (-0.01 %)
Spilled SGPRs: 39032 -> 39030 (-0.01 %)
Spilled VGPRs: 16326 -> 16326 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 744 -> 744 (0.00 %) dwords per thread
Code Size: 123755028 -> 123753316 (-0.00 %) bytes
Compile Time: 2751028 -> 2560786 (-6.92 %) milliseconds
LDS: 1415 -> 1415 (0.00 %) blocks
Max Waves: 972192 -> 972240 (0.00 %)
Wait states: 0 -> 0 (0.00 %)

vkpipeline-db results RADV (VEGA):

Totals from affected shaders:
SGPRS: 160 -> 160 (0.00 %)
VGPRS: 88 -> 88 (0.00 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 18268 -> 18152 (-0.63 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 26 -> 26 (0.00 %)
Wait states: 0 -> 0 (0.00 %)

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>