AMDGPU/GlobalISel: Add support for G_INTTOPTR

Summary: This is a no-op.

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, rovka, kristof.beyls, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D52916

llvm-svn: 343839