[CodeGenPrepare] Don't create inttoptr for ni ptrs

Summary:
Arguably non-integral pointers probably shouldn't show up here at all,
but since the backend doesn't complain and this takes valid (according
to the Verifier) IR and makes it invalid, make sure not to introduce
any inttoptr instructions if we're dealing with non-integral pointers.

Reviewed By: sanjoy
Differential Revision: https://reviews.llvm.org/D33110

llvm-svn: 306737