Add VerifyMhloBackendContract pass for MHLO.

[tanyokwok]

Does the VerifyMhloBackendContract pass exist? If not, could you also add it in another PR (see VerifyTosaBackendContract). This helps to make failures clearer when debugging, which is especially important for new contributors or other project maintainers who are not familiar with MHLO.

Originally posted by @silvasean in #1223 (comment)