[LICM] Reassociate GEPs to allow hoisting

Reassociate gep (gep ptr, idx1), idx2 to gep (gep ptr, idx2), idx1
if this would make the inner GEP loop invariant and thus hoistable.

This is intended to replace an InstCombine fold that does this (in
https://github.com/llvm/llvm-project/blob/04f61fb73dc6a994ab267d431f2fdaedc67430ff/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp#L2006).
The problem with the InstCombine fold is that LoopInfo is an optional
dependency, so it is not performed reliably.

Differential Revision: https://reviews.llvm.org/D146813