[SelectionDAG] Remove redundant stores more aggressively.

Handle TokenFactors more aggressively in
SDValue::reachesChainWithoutSideEffects.  This isn't really a
very effective change anymore because of other changes to
chain handling, but it's a cheap check, and the expanded
comments are still useful.

It might be possible to loosen the hasOneUse() requirement with a
deeper analysis, but a naive implementation of that check would be
expensive.

Differential Revision: https://reviews.llvm.org/D29845

llvm-svn: 298156