nir: evaluate if condition uses inside the if branches

nir: evaluate if condition uses inside the if branches

Since we know what side of the branch we ended up on we can just
replace the use with a constant.

All the spill changes in shader-db are from Dolphin uber shaders,
despite some small regressions the change is clearly positive.

V2: insert new constant after any phis in the
    use->parent_instr->type == nir_instr_type_phi path.

v3:
 - use nir_after_block_before_jump() for inserting const
 - check dominance of phi uses correctly

v4:
 - create some helpers as suggested by Jason.

v5 (Jason Ekstrand):
 - Use LIST_ENTRY to get the phi src

shader-db results IVB:

total instructions in shared programs: 9999201 -> 9993483 (-0.06%)
instructions in affected programs: 163235 -> 157517 (-3.50%)
helped: 132
HURT: 2

total cycles in shared programs: 231670754 -> 219476091 (-5.26%)
cycles in affected programs: 143424120 -> 131229457 (-8.50%)
helped: 115
HURT: 24

total spills in shared programs: 4383 -> 4370 (-0.30%)
spills in affected programs: 1656 -> 1643 (-0.79%)
helped: 9
HURT: 18

total fills in shared programs: 4610 -> 4581 (-0.63%)
fills in affected programs: 374 -> 345 (-7.75%)
helped: 6
HURT: 0

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>