AMDGPU/GlobalISel: Use ptrtoint to legalize constant 32-bit addrspacecast

This was trying to merge 2 32-bit pointers into a 64-bit pointer. The
artifact combiner was assuming merges to pointers use scalar sources,
and ended up inserting invalid bitcast from a pointer to a scalar. It
should probably be a verifier error to have pointer merge sources with
a pointer result.

Fixes verifier errors with EXPENSIVE_CHECKS.