[DAGCombiner] Unify Lifetime and memory Op aliasing.

Rework BaseIndexOffset and isAlias to fully work with lifetime nodes
and fold in lifetime alias analysis.

This is mostly NFC.

Reviewers: courbet

Reviewed By: courbet

Subscribers: hiraditya, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59794

llvm-svn: 357070