[DAG] Fold FrameIndex offset into BaseIndexOffset analysis. NFCI.

Relanding after restricting equalBaseIndex to not erroneuosly consider
a FrameIndices stemming from alloca from being comparable as its
offset is set post-selectionDAG.

Pull FrameIndex comparision reasoning from DAGCombiner::isAlias to
general BaseIndexOffset.

llvm-svn: 306688