[MDBuilder] `mergeCallbackEncodings` fails due to inspecting the wrong node (#92466)

Given the following metadata as, with `!6` as `ExistingCallbacks` and
`!8` as `NewCB`:
```
!6 = !{!7}
!7 = !{i64 0, i1 false}
!8 = !{i64 2, i64 3, i1 false}
```
The merge function should add `!8` to the list of `!6`, i.e. `!6 =
!{!7,!8}`. However, at the moment the check if this is legal, tries to
interpret `!7` as integer instead of it's operand.