[llvm] [InstrRef] Preserve debug instr num in aarch64-ldst-opt. (PR #136009)

[Jeremy Morse via llvm-commits]

================
@@ -965,6 +965,28 @@ static void updateDefinedRegisters(MachineInstr &MI, LiveRegUnits &Units,
       Units.addReg(MOP.getReg());
 }
 
+/// This function will add a new entry into the debugValueSubstitutions table
+/// when two instruction have been merged into a new one represented by \p
+/// MergedInstr.
+static void addDebugSubstitutionsToTable(MachineFunction *MF,
+                                         unsigned InstrNumToSet,
+                                         MachineInstr &OriginalInstr,
+                                         MachineInstr &MergedInstr) {
+
+  // Figure out the Operand Index of the destination register of the
+  // OriginalInstr in the new MergedInstr.
+  auto Reg = OriginalInstr.getOperand(0).getReg();
+  unsigned OperandNo = 0;
+  for (auto Op : MergedInstr.operands()) {
----------------
jmorse wrote:

```suggestion
  for (const auto &Op : MergedInstr.operands()) {
```
Avoids the risk of copy-constructing a MachineOperand where we can just refer to it.

https://github.com/llvm/llvm-project/pull/136009