[Scheduling] Fall back to the fast cluster algorithm if the DAG is too complex

We have added a new load/store cluster algorithm in D85517. However, AArch64 see
some compiling deg with the new algorithm as the IsReachable() is not cheap if
the DAG is complex. O(M+N) See https://bugs.llvm.org/show_bug.cgi?id=47966
So, this patch added a heuristic to switch to old cluster algorithm if the DAG is too complex.

Reviewed By: Owen Anderson

Differential Revision: https://reviews.llvm.org/D90144