[llvm] [AMDGPU][NFC] Refactor SCC optimization (PR #165871)

[Matt Arsenault via llvm-commits]

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@@ -10618,6 +10618,42 @@ bool SIInstrInfo::analyzeCompare(const MachineInstr &MI, Register &SrcReg,
   return false;
 }
 
+// SCC is already valid after SCCValid.
+// SCCRedefine will redefine SCC to the same value already available after
+// SCCValid. If there are no intervening SCC conflicts delete SCCRedefine and
+// update kill/dead flags if necessary.
+static bool optimizeSCC(MachineInstr *SCCValid, MachineInstr *SCCRedefine,
+                        const SIRegisterInfo *RI) {
+  MachineInstr *KillsSCC = nullptr;
+  for (MachineInstr &MI : make_range(std::next(SCCValid->getIterator()),
+                                     SCCRedefine->getIterator())) {
+    if (MI.modifiesRegister(AMDGPU::SCC, RI))
+      return false;
+    if (MI.killsRegister(AMDGPU::SCC, RI))
+      KillsSCC = &MI;
+  }
+  if (MachineOperand *SccDef =
+          SCCValid->findRegisterDefOperand(AMDGPU::SCC, /*TRI=*/nullptr))
+    SccDef->setIsDead(false);
+  if (KillsSCC)
+    KillsSCC->clearRegisterKills(AMDGPU::SCC, /*TRI=*/nullptr);
+  SCCRedefine->eraseFromParent();
+  return true;
+}
+
+static bool foldableSelect(MachineInstr *Def) {
+  if (Def->getOpcode() == AMDGPU::S_CSELECT_B32 ||
+      Def->getOpcode() == AMDGPU::S_CSELECT_B64) {
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arsenm wrote:

Early return and reduce indent 

https://github.com/llvm/llvm-project/pull/165871