[InstCombine] Fix inbounds preservation when merging GEPs (PR55722)

Even if the total offset is inbounds, we might represent it by first
performing a large negative offset and then a small positive one.
With inbounds semantics as currently specified, each offset must
be inbounds individually, not just the overall offset of the GEP.

Fix this by checking that the sign of all offsets is the same.

Fixes https://github.com/llvm/llvm-project/issues/55722.