[llvm] [TableGen] Add sub-register overflow tests for exact-fit and non-covered registers (PR #210529)

[Rahul Joshi via llvm-commits]

jurahul wrote:

Note that LLVM's AI tool policy recommended that PR descriptions be written by hand partly to avoid too verbose descriptions generated by AI. This one seems too verbose for what it does. Can it be made more concise?

https://github.com/llvm/llvm-project/pull/210529