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

[Kunqiu Chen via llvm-commits]

Camsyn wrote:

> Should this be the default behaviour (with a --version bump)?

This change is the default behaviour, but only enabled when an extra UTC option `--check-comments` specified. 
And I don't think it's necessary to control it with versions.

> Being able to use CHECK-NEXT more instead of the more fragile CHECK is IMO a good thing.
This change is not introduced by this PR. I think it using `CHECK` instead of `CHECK-NEXT` is somehow reasonable, since there is an empty line between the two lines required checking:
```LLVM
; CHECK-NEXT:    br label [[TMP1:%.*]]
; CHECK:         [[TMP2:%.*]] = load i32, ptr @d, align 4
....
  br label %1

; <label>:1:                                      ; preds = %17, %0
  %2 = load i32, ptr @d, align 4
```

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