AMDGPU/InsertWaitcnts: Remove kill-related logic

Summary:
This is not needed, because we don't actually insert relevant branches
for KILLs that late in the compilation flow.

Besides, this was always checking for the wrong kill opcode anyway...

Reviewers: msearles, rampitec, scott.linder, kanarayan

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

Differential Revision: https://reviews.llvm.org/D54085

llvm-svn: 346362