AMDGPU/GlobalISel: Implement select() for @llvm.amdgcn.cvt.pkrtz

Reviewers: arsenm, nhaehnle

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D45907

llvm-svn: 334757