[llvm] [UTC] Support to test annotated IR (PR #165419)

[Kunqiu Chen via llvm-commits]

Camsyn wrote:

> Technically I guess the whole file is IR but I can't think of a better name that isn't super long (--check-comments-in-function). So this sounds good to me.

My mistake. It should be comments attached to IR instructions.

I'm wondering if we need to check all comments through this option, i.e., global/function/BB/instruction comments.
If not, maybe `--check-inst-comments` is a good name, and I will validate that it only works on IR instructions.

https://github.com/llvm/llvm-project/pull/165419