[DAGCombine] Improve alias analysis for chain of independent stores.

FindBetterNeighborChains simulateanously improves the chain
dependencies of a chain of related stores avoiding the generation of
extra token factors. For chains longer than the GatherAllAliasDepths,
stores further down in the chain will necessarily fail, a potentially
significant waste and preventing otherwise trivial parallelization.

This patch directly parallelize the chains of stores before improving
each store. This generally improves DAG-level parallelism.

Reviewers: courbet, spatel, RKSimon, bogner, efriedma, craig.topper, rnk

Subscribers: sdardis, javed.absar, hiraditya, jrtc27, atanasyan, llvm-commits

Differential Revision: https://reviews.llvm.org/D53552

llvm-svn: 346432