[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. I'll add an assert for that.

aha! It's that a tied operands are generally a def (output) followed by a use (input); if the input/use is added without a corresponding def, MRI has conniptions.

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