DebugInfo: Add test coverage for non-zero addrx in DW_FORM_LLVM_addrx_offset

Noticed in code review
https://github.com/llvm/llvm-project/commit/4318028cd2d7633a0cdeb0b5d4d2ed81fab87864#commitcomment-57738034

But the issue had already been fixed in
943b3048 due to a code checking tool
(PVS studio) identification, but that lacked test coverage.

Refactor this test a little bit too by using more CHECK-SAME to help the
checks fail sooner (rather than, if the addrx or sizes are wrong, having
that check bind to a much later output line - and then fail due to the
implicit-check-nots, which don't provide a lot of information about
where the intended check was likely to land) & more informatively.