[AMDGPU] Don't remove short branches over kills

Summary:
D68092 introduced a new SIRemoveShortExecBranches optimization pass and
broke some graphics shaders. The problem is that it was removing
branches over KILL pseudo instructions, and the fix is to explicitly
check for that in mustRetainExeczBranch.

Reviewers: critson, arsenm, nhaehnle, cdevadas, hakzsam

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73771

Change-Id: I84d96d165c4f6da4870e125d1b2664b9daad51e0

llvm/lib/Target/AMDGPU/SIRemoveShortExecBranches.cpp

@@ -96,6 +96,9 @@ bool SIRemoveShortExecBranches::mustRetainExeczBranch(
       if (TII->hasUnwantedEffectsWhenEXECEmpty(*I))
         return true;
 
+      if (TII->isKillTerminator(I->getOpcode()))
+        return true;
+
       // These instructions are potentially expensive even if EXEC = 0.
       if (TII->isSMRD(*I) || TII->isVMEM(*I) || TII->isFLAT(*I) ||
           I->getOpcode() == AMDGPU::S_WAITCNT)