nir: Use 8-bit types for most info fields
This shrinks nir_intrinsics.c.o from 73K to 35K and nir_opcodes.c.o from 64K to 31K on a release build.
This shrinks nir_intrinsics.c.o from 73K to 35K and nir_opcodes.c.o from 64K to 31K on a release build.