static analyzer: Further reduce the analyzer's memory usage when analyzing...

static analyzer: Further reduce the analyzer's memory usage when analyzing sqlite3 by 7-10% by recylcing "uninteresting" ExplodedNodes.

The optimization involves eagerly pruning ExplodedNodes from the ExplodedGraph that contain
practically no difference between the predecessor and successor nodes.  For example, if
the state is different between a predecessor and a node, the node is left in.  Only for
the 'environment' component of the state do we not care if the ExplodedNodes are different.
This paves the way for future optimizations where we can reclaim the environment objects.

llvm-svn: 125154