[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306688 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed