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%.

llvm-svn: 98144