[DebugInfo] Ensure that a demanded bits optimisation in

InstCombine does not result in an incorrect debuginfo variable
value

- Add an additional salvage and a test.

Reviewers: aprantl, djtodoro

Differential Revision: https://reviews.llvm.org/D76854

Bugzilla:  https://bugs.llvm.org/show_bug.cgi?id=44371