[AMDGPU] Omit KILL instructions from hazard recognizer
In some cases the KILL was causing a hazard to be introduced as these were
scheduled into hazard slots, but don't result in an instruction.
KILL shouldn't be considered for hazard recognition.
Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, tpr, t-tye, llvm-commits
Differential Revision: https://reviews.llvm.org/D58898