I've thought this should at minimum work for all the legal 32-bit
types for a long time. Arguably we should also legalize other types,
or at least multiples of 32.
This fixes selection for non-s32 types when readfirstlane is inserted
for SGPR return values.
This will require API users to add the type mangling when getting the
declaration. I think the way mesa constructs intrinsics will happen to
get away without needing to change anything.
Hoist this above the "if" on line 605?