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

[Matthias Braun via llvm-commits]

MatzeB wrote:

I'd be extremely surprised if we mess up the assertion setting for unit tests.

Have you tried adding a call to `verifyMachineFunction()` yet? Sometimes that uncovers more problems...

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