[InstCombine] When swapping GEPs, only keep inbounds if both are

If only one of the GEPs is inbounds, then after swapping, there is
no guarantee that one of them will be inbounds as well
(see e.g. https://alive2.llvm.org/ce/z/agaCnp).

This is only a partial fix, because even if both are inbounds, the
result is not necessarily inbounds (if the offsets have different
signs).