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

[Nick Desaulniers via llvm-commits]

================
@@ -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;
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nickdesaulniers wrote:

Why isn't `SmallDenseMap` listed in [ADT](https://llvm.org/docs/ProgrammersManual.html#map-like-containers-std-map-densemap-etc)?

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