[DAGCombine] Limit the number of times for the same store and root nodes

to bail out in store merging dependence check.

We run into a case where dependence check in store merging bail out many times
for the same store and root nodes in a huge basicblock. That increases compile
time by almost 100x. The patch add a map to track how many times the bailing
out happen for the same store and root, and if it is over a limit, stop
considering the store with the same root as a merging candidate.

Differential Revision: https://reviews.llvm.org/D65174

llvm-svn: 367472