[AMDGPU][Waitcnt] Update a few tests to use default waitcnt pass...

[AMDGPU][Waitcnt] Update a few tests to use default waitcnt pass (si-insert-waitcnts) rather than old pass (si-insert-waits); this is a small step towards the overall goal of removing the old waitcnt pass, which is no longer maintained.

Differential Revision: https://reviews.llvm.org/D46154

llvm-svn: 331062