IR verifier does not enforce metadata structure of read_register/write_register operands

This fix should not have been nececessary: https://github.com/llvm/llvm-project/pull/162414

The verifier accepts a raw MDString reference. That would be a better format, maybe we should auto upgrade to that

Start by reviewing the linked pull request and the IR verifier handling for read_register/write_register operands. Determine the required metadata structure and whether raw MDString references should be accepted or upgraded; done means the verifier rejects invalid operands and the intended compatibility behavior is covered.

Written by the indexing model from the issue text.