aco: optimize AC_FETCH_FORMAT_SNORM alpha adjust
This looks like it was copied from LLVM, which didn't have a fmax intrinsic.
fossil-db (GFX8):
Totals from 43 (0.03% of 140385) affected shaders:
CodeSize: 49660 -> 49488 (-0.35%)
Instrs: 10434 -> 10391 (-0.41%)
Cycles: 41736 -> 41564 (-0.41%)
VMEM: 13793 -> 13754 (-0.28%)