[AMDGPU] Omit KILL instructions from hazard recognizer

Summary:
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.

Change-Id: Ib6d2a2160f8c94cd0ce611ab198c7e4f46aeffcf

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58898

llvm-svn: 355384