nir/gcm: fix pushing instructions into if blocks

The previous logic would just set the block to the instructions original location if we couldn't evict it from a loop.

For now we only push const loads to a later block inside ifs but we can add more heuristics later. This change helps a hand full of shaders but also stops a CTS regression caused by excess spilling after a series I'm working on to disables more of the GLSL IR optimisation passes.

Shader-db results iris (BDW):

total instructions in shared programs: 17529759 -> 17529749 (<.01%)
instructions in affected programs: 15929 -> 15919 (-0.06%)
helped: 5
HURT: 2
helped stats (abs) min: 1 max: 5 x̄: 2.40 x̃: 2
helped stats (rel) min: 0.06% max: 0.15% x̄: 0.11% x̃: 0.12%
HURT stats (abs)   min: 1 max: 1 x̄: 1.00 x̃: 1
HURT stats (rel)   min: 0.06% max: 0.06% x̄: 0.06% x̃: 0.06%
95% mean confidence interval for instructions value: -3.34 0.49
95% mean confidence interval for instructions %-change: -0.14% 0.02%
Inconclusive result (value mean confidence interval includes 0).

total cycles in shared programs: 861109994 -> 861099681 (<.01%)
cycles in affected programs: 7027698 -> 7017385 (-0.15%)
helped: 95
HURT: 72
helped stats (abs) min: 1 max: 7995 x̄: 138.54 x̃: 9
helped stats (rel) min: <.01% max: 15.96% x̄: 0.54% x̃: 0.11%
HURT stats (abs)   min: 1 max: 474 x̄: 39.56 x̃: 12
HURT stats (rel)   min: <.01% max: 1.17% x̄: 0.20% x̃: 0.11%
95% mean confidence interval for cycles value: -159.05 35.54
95% mean confidence interval for cycles %-change: -0.45% 0.01%
Inconclusive result (value mean confidence interval includes 0).

total spills in shared programs: 17606 -> 17605 (<.01%)
spills in affected programs: 323 -> 322 (-0.31%)
helped: 1
HURT: 0

total fills in shared programs: 22599 -> 22598 (<.01%)
fills in affected programs: 1348 -> 1347 (-0.07%)
helped: 1
HURT: 0