AMDGPU/InsertWaitcnts: Cleanup some old cruft (NFCI)

Summary: Remove redundant logic and simplify control flow.

Reviewers: msearles, rampitec, scott.linder, kanarayan

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

Differential Revision: https://reviews.llvm.org/D54086

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346363 91177308-0d34-0410-b5e6-96231b3b80d8