[llvm] [X86] Add combine tests for pointers of mixed sizes (NFC) (PR #68219)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon approved this pull request.

LGTM as precommit for #67168 to fix #66873 (please reference these in the commit message)

https://github.com/llvm/llvm-project/pull/68219