[llvm] [AMDGPU] In instruction selector, allow copy from physical reg to s1 (PR #96157)

[Matt Arsenault via llvm-commits]

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@@ -131,6 +131,13 @@ bool AMDGPUInstructionSelector::selectCOPY(MachineInstr &I) const {
   Register SrcReg = Src.getReg();
 
   if (isVCC(DstReg, *MRI)) {
+    // Allow copy from physical register other than SCC to s1.
+    if (SrcReg.isPhysical() && SrcReg != AMDGPU::SCC) {
+      const TargetRegisterClass *DstRC = MRI->getRegClassOrNull(DstReg);
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arsenm wrote:

Shouldn't be re-querying the class of DstReg, isVCC should tell you everything you need to know here 

https://github.com/llvm/llvm-project/pull/96157