[DAG] Bound loop dependence check in merge optimization.

The loop dependence check looks for dependencies between store merge
candidates not captured by the chain sub-DAG doing a check of
predecessors which may be very large. Conservatively bound number of
nodes checked for compilation time. (Resolves PR34326).

Landing on behalf of Nirav Dave to unblock the 5.0.0 release.

Differential Revision: https://reviews.llvm.org/D37220

llvm-svn: 312022