Really apply (unnoticed weird git-svn merge conflict in 98144): Refactor RegionStore...

Really apply (unnoticed weird git-svn merge conflict in 98144): Refactor RegionStore::RemoveDeadBindings to use the same core cluster analysis algorithm as RegionStore::InvalidateRegions().

Beyond simplifying the algorithm significantly, we no longer
need to build subregion maps in RemoveDeadBindings().  This
and other changes cause a significant speedup: the time to
analyze sqlite3.c (single core) drops by 14%.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@98159 91177308-0d34-0410-b5e6-96231b3b80d8