[InstCombine] Preserve inbounds when canonicalizing gep+add (#90160)

When canonicalizing gep+add into gep+gep we can preserve inbounds if the
add is also nsw and both add operands are non-negative (or both
negative, but I don't think that's practically relevant).

Proof: https://alive2.llvm.org/ce/z/tJLBta