nir: Make intrinsic flags into an enum
nir: Make intrinsic flags into an enum

This should be much better for debugging as GDB will pick up on the fact
that it's an enum and actually tell you what you're looking at instead of
giving you some arbitrary hex value you have to go look up.

Reviewed-by: Connor Abbott <71178acffcc112b21e5858656e5751f5e4aa9364@gmail.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler