radv: make use of nir_lower_load_const_to_scalar()

radv: make use of nir_lower_load_const_to_scalar()

This allows NIR to CSE more operations. LLVM does this also so the
impact is limited, however doing this in NIR allows other opts to
make progress. For example in radeonsi more loops are unrolled in
Civilization Beyond Earth.

The actual pipeline-db stats are not overwhelming but even in the
negatively affected shaders the NIR is clearly better. It just
happens that the code shuffling and in some cases calls to max
rather than a flt result in the final output from LLVM not
giving as good numbers.

However this is an incremental opt that further passes build off
so the change should be made IMO.

Totals from affected shaders:
SGPRS: 20192 -> 20184 (-0.04 %)
VGPRS: 19516 -> 19524 (0.04 %)
Spilled SGPRs: 437 -> 444 (1.60 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 1527444 -> 1522276 (-0.34 %) bytes
LDS: 6 -> 6 (0.00 %) blocks
Max Waves: 1018 -> 1016 (-0.20 %)
Wait states: 0 -> 0 (0.00 %)

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>