[DAG] Prune cycle check in store merge.

As part of merging stores we check that fusing the nodes does not
cause a cycle due to one candidate store being indirectly dependent on
another store (this may happen via chained memory copies). This is
done by searching if a store is a predecessor to another store's
value.

Prune the search at the candidate search's root node which is a
predecessor to all candidate stores. This reduces the
size of the subgraph searched in large basic blocks.

Reviewers: jyknight

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D46955

llvm-svn: 332490