[AMDGPU] Add ISD::FSHR Handling to AMDGPUISD::PERM matching

Pulled out of D159533, which encourages (zext (trunc x)) -> x folds, leading to more ISD::FSHR nodes, which was breaking some existing AMDGPUISD::PERM tests

Differential Revision: https://reviews.llvm.org/D159533