[InstSimplify] Fold icmp of allocas based on offset difference

Strengthen the fold for icmps of non-overlapping storage, by
working on the difference of offsets, rather than considering
both offsets independently. In particular, this allows handling
comparisons of pointers to the end of equal-sized allocations.

Proofs: https://alive2.llvm.org/ce/z/Po2nL4

Differential Revision: https://reviews.llvm.org/D153752