[AMDGPU] Fix bugs in new waitcnt pass. Add test.

- new waitcnt pass remains off by default; -enable-si-insert-waitcnts=1 to enable it
- fix handling of PERMUTE ops
- fix insertion of waitcnt instrs at function begin/end ( port of analogous code that was added to old waitcnt pass )
- add new test

  Differential Revision: https://reviews.llvm.org/D33114

llvm-svn: 304311