AMDGPU/SI: Do not generate s_waitcnt after ds_permute/ds_bpermute

Symmary:
  ds_permute/ds_bpermute do not read memory so s_waitcnt is not needed.

Reviewers
  arsenm, tstellarAMD

Subscribers
  llvm-commits, arsenm

Differential Revision:
  http://reviews.llvm.org/D18197

llvm-svn: 263720