iris: Bypass half-float pack/unpack lowering.

iris: Bypass half-float pack/unpack lowering.

This skips GLSL IR lowering of pack/unpackHalf operations, allowing
the NIR optimizer to see them

Improves performance in Synmark2's OglCSDof by about 2x, by cutting
about 90% of the cycles from one of the compute shaders.

shader-db statistics on Skylake:

4 compute shaders went from SIMD8 to SIMD16.

total instructions in shared programs: 15598871 -> 15542568 (-0.36%)
instructions in affected programs: 143016 -> 86713 (-39.37%)
helped: 144
HURT: 0
helped stats (abs) min: 17 max: 4669 x̄: 390.99 x̃: 164
helped stats (rel) min: 7.48% max: 85.28% x̄: 30.17% x̃: 24.22%
95% mean confidence interval for instructions value: -510.50 -271.49
95% mean confidence interval for instructions %-change: -32.70% -27.65%
Instructions are helped.

total cycles in shared programs: 371973958 -> 368902103 (-0.83%)
cycles in affected programs: 5557722 -> 2485867 (-55.27%)
helped: 144
HURT: 0
helped stats (abs) min: 106 max: 1026600 x̄: 21332.33 x̃: 1697
helped stats (rel) min: 0.53% max: 88.98% x̄: 36.12% x̃: 34.67%
95% mean confidence interval for cycles value: -41570.02 -1094.64
95% mean confidence interval for cycles %-change: -38.44% -33.80%
Cycles are helped.

total spills in shared programs: 11936 -> 11903 (-0.28%)
spills in affected programs: 110 -> 77 (-30.00%)
helped: 3
HURT: 2

total fills in shared programs: 25644 -> 25178 (-1.82%)
fills in affected programs: 677 -> 211 (-68.83%)
helped: 5
HURT: 0

total loops in shared programs: 4830 -> 4829 (-0.02%)
loops in affected programs: 1 -> 0
helped: 1
HURT: 0