[llvm] AMDGPU: Mark the SCC def dead when expanding 64-bit scalar add/sub (PR #226269)

[Matt Arsenault via llvm-commits]

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@@ -5707,6 +5707,7 @@ static MachineBasicBlock *expand64BitScalarArithmetic(MachineInstr &MI,
   MachineOperand &Src1 = MI.getOperand(2);
   bool IsAdd = (MI.getOpcode() == AMDGPU::S_ADD_U64_PSEUDO);
   if (ST.hasScalarAddSub64()) {
+    // FIXME: If scc is used, this deletes the def
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arsenm wrote:

My preferred solution would be to not use the pseudo in the first place if the instruction is available 

https://github.com/llvm/llvm-project/pull/226269