[InstCombine] Fix incorrect inbounds on GEP of GEP (PR44425)

This fixes https://bugs.llvm.org/show_bug.cgi?id=44425. We need to
drop inbounds if one of the GEPs is not inbounds. This was already
done when creating a new GEP, but not when modifying in place.

Differential Revision: https://reviews.llvm.org/D72059