radeonsi/nir: 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 some loops in Civilization Beyond Earth
shaders are unrolled.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>