Add option to 0 init variable in shaders
- the first one is from !1769 (closed) - @alyssa would you mind merging it? (if you don't want this commit I can copy the
nir_undef_to_zerofunction only for radeonsi).
- the second adds a AMD_DEBUG=
zeroinitoption to use
nir_undef_to_zeroafter other optimizations (maybe it should be run before? not sure)