[llvm] [MachineInstr] add insert method for variadic instructions (PR #67699)

[Matthias Braun via llvm-commits]

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@@ -2475,3 +2475,45 @@ MachineInstr::getFirst5RegLLTs() const {
       Reg2, getRegInfo()->getType(Reg2), Reg3, getRegInfo()->getType(Reg3),
       Reg4, getRegInfo()->getType(Reg4));
 }
+
+void MachineInstr::insert(mop_iterator It, ArrayRef<MachineOperand> Ops) {
+  if (!It || Ops.empty())
+    return;
+
+  // Do one pass to untie operands.
+  DenseMap<unsigned, unsigned> TiedOpIndices;
+  for (const MachineOperand &MO : operands()) {
+    if (MO.isReg() && MO.isTied()) {
+      unsigned OpNo = getOperandNo(&MO);
+      unsigned TiedTo = findTiedOperandIdx(OpNo);
+      TiedOpIndices[OpNo] = TiedTo;
+      untieRegOperand(OpNo);
+    }
+  }
+
+  assert(It->getParent() == this && "iterator points to operand of other inst");
+  const unsigned OpIdx = getOperandNo(It);
+  const unsigned NumOperands = getNumOperands();
+  const unsigned OpsToMove = NumOperands - OpIdx;
+
+  SmallVector<MachineOperand> MovingOps;
+  MovingOps.reserve(OpsToMove);
+
+  for (unsigned I = 0; I < OpsToMove; ++I) {
+    MovingOps.emplace_back(getOperand(OpIdx));
+    removeOperand(OpIdx);
----------------
MatzeB wrote:

Maybe do this in reverse order so we waste less cycles on the `moveOperands()` call within `MachineInstr::removeOperand()`.   On that note I wonder if `moveOperands()` would work here as well instead of the `removeOperand` / `addOperand` for `MovingOps` here?

Admittedly it looks like `moveOperands()` doesn't really adjust the `TiedTo` markers so that's why it wouldn't work here?

https://github.com/llvm/llvm-project/pull/67699