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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> Spooky, looks like my newly added tests are failing in presubmit.

The call to `getRegUseDefListHead` in `MachineRegisterInfo::addRegOperandToUseList` is returning nullptr.  I must have to inform MRI of the new reg BEFORE adding it as an operand.

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