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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

Added to the description:

> This is meant to mirror MCInst::insert.

Also, should I limit the new ops to not be "implicit" ops? Looking at `MachineInstr::addOperand`, it seems like where they are located is not in a specific ordering relative to the "explicit" ops.

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