nir: Make the type casting operations static inline functions

nir: Make the type casting operations static inline functions

Previously, the casting operations were macros.  While this is usually
fine, the casting macro used the input parameter twice leading to strange
behavior when you passed the result of another function into it.  Since we
know the source and destination types explicitly, we don't loose anything
by making it a function.

Also, this gives us a nice little macro for creating cast function that
will hopefully prevent mistyping.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>