spirv: Change vtn_null_constant() to use vtn_type

spirv: Change vtn_null_constant() to use vtn_type

This is a preparation to handle OpConstantNull for pointers, we'll use
the vtn_type to get to the address format and then the appropriate
representation of NULL pointer.

v2: Move rest of body to use vtn_type. (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>