nir/large_constants: Use dominance information to find more constants

nir/large_constants: Use dominance information to find more constants

Relax the restriction that all the writes need to be in the first
block: now accept variables that have all the writes in the same
block, and all the reads are dominated by that block.

This let the pass identify large constants that are local to a helper
function.  The writes will be at the place that the function is
inlined, possibly not in the first block (but still all in the same
block).

Results for vkpipeline-db in SKL:

total instructions in shared programs: 3624891 -> 3623145 (-0.05%)
instructions in affected programs: 79416 -> 77670 (-2.20%)
helped: 16
HURT: 0

total cycles in shared programs: 1458149667 -> 1458147273 (<.01%)
cycles in affected programs: 30154164 -> 30151770 (<.01%)
helped: 14
HURT: 2

total loops in shared programs: 2437 -> 2437 (0.00%)
loops in affected programs: 0 -> 0
helped: 0
HURT: 0

total spills in shared programs: 8813 -> 8745 (-0.77%)
spills in affected programs: 2894 -> 2826 (-2.35%)
helped: 8
HURT: 0

total fills in shared programs: 23470 -> 23392 (-0.33%)
fills in affected programs: 12248 -> 12170 (-0.64%)
helped: 6
HURT: 2

LOST:   0
GAINED: 0

Results for shader-db in SKL with Iris:

total instructions in shared programs: 15379442 -> 15379392 (<.01%)
instructions in affected programs: 837 -> 787 (-5.97%)
helped: 2
HURT: 2
helped stats (abs) min: 27 max: 27 x̄: 27.00 x̃: 27
helped stats (rel) min: 10.47% max: 10.67% x̄: 10.57% x̃: 10.57%
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 1.23% max: 1.23% x̄: 1.23% x̃: 1.23%
95% mean confidence interval for instructions value: -39.14 14.14
95% mean confidence interval for instructions %-change: -15.51% 6.17%
Inconclusive result (value mean confidence interval includes 0).

total loops in shared programs: 4880 -> 4880 (0.00%)
loops in affected programs: 0 -> 0
helped: 0
HURT: 0

total cycles in shared programs: 370677237 -> 370676567 (<.01%)
cycles in affected programs: 17852 -> 17182 (-3.75%)
helped: 2
HURT: 1
helped stats (abs) min: 338 max: 356 x̄: 347.00 x̃: 347
helped stats (rel) min: 13.98% max: 14.64% x̄: 14.31% x̃: 14.31%
HURT stats (abs)   min: 24 max: 24 x̄: 24.00 x̃: 24
HURT stats (rel)   min: 0.18% max: 0.18% x̄: 0.18% x̃: 0.18%

total spills in shared programs: 11772 -> 11772 (0.00%)
spills in affected programs: 0 -> 0
helped: 0
HURT: 0

total fills in shared programs: 24948 -> 24948 (0.00%)
fills in affected programs: 0 -> 0
helped: 0
HURT: 0

LOST:   0
GAINED: 0

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