[analyzer] Recover all PreStmtPurgeDeadSymbols nodes with a single successor or predecessor.

These nodes are never consulted by any analyzer client code, so they are
used only for machinery for removing dead bindings.  Once successor nodes
are generated they can be safely removed.

This greatly reduces the amount of nodes that are generated in some case,
lowering the memory regression when analyzing Sema.cpp introduced by
r176010 from 14% to 2%.