nir: Return nir_type_invalid for non-numeric base types

nir: Return nir_type_invalid for non-numeric base types

Now that the type gathering function look at instructions that might
have other types, return invalid type instead of crashing.  That
invalid will be properly ignored later.

Fixes: c12750527b7 "nir: add type information to load uniform/input and store output intrinsics"
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>