AMDGPU/GlobalISel: Select amdgcn.cvt.pkrtz to 64-bit instructions

Summary: The 32-bit variants do not exist on VI+.

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/D52958

llvm-svn: 343985