[llvm] AMDGPU: Refactor three-address conversion (NFC) (PR #162558)

[Matt Arsenault via llvm-commits]

================
@@ -4080,6 +4083,58 @@ MachineInstr *SIInstrInfo::convertToThreeAddress(MachineInstr &MI,
           UpdateDefIndex(SR);
       }
     }
+  }
+
+  if (U.RemoveMIUse) {
+    MachineRegisterInfo &MRI = MBB.getParent()->getRegInfo();
+    // The only user is the instruction which will be killed.
+    Register DefReg = U.RemoveMIUse->getOperand(0).getReg();
+
+    if (MRI.hasOneNonDBGUse(DefReg)) {
+      // We cannot just remove the DefMI here, calling pass will crash.
+      U.RemoveMIUse->setDesc(get(AMDGPU::IMPLICIT_DEF));
+      U.RemoveMIUse->getOperand(0).setIsDead(true);
+      for (unsigned I = U.RemoveMIUse->getNumOperands() - 1; I != 0; --I)
+        U.RemoveMIUse->removeOperand(I);
+      if (LV)
+        LV->getVarInfo(DefReg).AliveBlocks.clear();
+    }
+
+    if (LIS) {
+      LiveInterval &DefLI = LIS->getInterval(DefReg);
+
+      // We cannot delete the original instruction here, so hack out the use
+      // in the original instruction with a dummy register so we can use
+      // shrinkToUses to deal with any multi-use edge cases. Other targets do
+      // not have the complexity of deleting a use to consider here.
+      Register DummyReg = MRI.cloneVirtualRegister(DefReg);
+      for (MachineOperand &MIOp : MI.uses()) {
+        if (MIOp.isReg() && MIOp.getReg() == DefReg) {
----------------
arsenm wrote:

Preexisting, but this looks broken for subregisters

https://github.com/llvm/llvm-project/pull/162558