[GlobalOpt] Handle non-instruction MTI source (PR54572)

This was reusing a cast to GlobalVariable to check for an
Instruction, which means we'll try to dereference a null pointer
if it's not actually a GlobalVariable. We should be casting
MTI->getSource() instead.

I don't think this problem is really specific to opaque pointers,
but it certainly makes it a lot easier to reproduce.

Fixes https://github.com/llvm/llvm-project/issues/54572.