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

[Matthias Braun via llvm-commits]

https://github.com/MatzeB commented:

Adding an API to insert operands in the middle of the operand list seems fine. I'd just leave semantics like implicit, variadic etc. out of it (and make that a problem for callers and verifiers).

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