[llvm] AMDGPU: Extract mergeIncomingLaneMasks from SILowerI1Copies lowerPhis (PR #216995)

[Matt Arsenault via llvm-commits]

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@@ -449,9 +449,92 @@ AMDGPU::PhiLoweringHelper::PhiLoweringHelper(MachineFunction *MF,
   TII = ST->getInstrInfo();
 }
 
-bool AMDGPU::PhiLoweringHelper::lowerPhis() {
+void AMDGPU::PhiLoweringHelper::mergeIncomingLaneMasks(
+    Register DstReg, MachineBasicBlock &MBB,
+    SmallVectorImpl<Incoming> &Incomings, MachineIDFSSAUpdater &SSAUpdater) {
   LoopFinder LF(*DT, *PDT);
   PhiIncomingAnalysis PIA(*PDT, TII);
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arsenm wrote:

I assumed these were cheap because of the intiailize in the loop, but I suppose that PrevMBB stuff was to only call this once 

https://github.com/llvm/llvm-project/pull/216995