AMDGPU: Add patterns for v4i16/v4f16 -> v4i16/v4f16 bitcasts

Reviewers: arsenm, tstellar

Reviewed By: arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D55058

llvm-svn: 349694