Avoid removing physreg uses in MachineDCE with LIS present

[rampitec]

Summary

One of OOT targets hits "Live segment doesn't end at a valid instruction"
error if physreg use is removed. I do not have a reproducer for that or
access to the target, but that seems to be reasonable to avoid removing
dead phys reg uses for the same reason as dead defs.

lib/CodeGen/DeadMachineInstructionElim.cpp

bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const {
   // Don't delete instructions with side effects.
   bool SawStore = false;
   if (!MI->isSafeToMove(nullptr, SawStore) && !MI->isPHI())
     return false;
 
   // Examine each operand.
   for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
     const MachineOperand &MO = MI->getOperand(i);
-    if (MO.isReg() && MO.isDef()) {
-      unsigned Reg = MO.getReg();
-      if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
-        // Don't delete live physreg defs, or any reserved register defs.
-        // Do not remove physreg defs if we have LIS as we may be unable
-        // to accurately recompute its liveness.
-        if (LivePhysRegs.test(Reg) || MRI->isReserved(Reg) || LIS)
-          return false;
-      } else {
-        // An instruction can also use its def in case if it is a tied operand.
-        // TODO: Technically we can also remove it if def dominates the use.
-        //       This can happen when two instructions define different subregs
-        //       of the same register.
-        for (const MachineInstr &Use : MRI->use_nodbg_instructions(Reg)) {
-          if (&Use != MI)
-            // This def has a non-debug use. Don't delete the instruction!
-            return false;
-        }
-      }
-    }
-  }
+    if (!MO.isReg())
+      continue;
+
+    // Do not remove physreg defs or uses if we have LIS as we may be unable
+    // to accurately recompute its liveness.
+    if (LIS && TargetRegisterInfo::isPhysicalRegister(Reg) &&
+        MRI->isAllocatable(Reg))
+      return false;
+
+    if (!MO.isDef())
+      continue;
+
 
   // If there are no defs with uses, the instruction is dead.
   return true;
 }
 
 bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
   if (skipFunction(MF.getFunction()))
     return false;