[analyzer] add the notion of an "interesting" lvalue expression for ExplodedNode pruning.

r175988 modified the ExplodedGraph trimming algorithm to retain all
nodes for "lvalue" expressions.  This patch refines that notion to
only "interesting" expressions that would be used for diagnostics.
[analyzer] Relax assumption in FindLastStoreBRVisitor that the thing we are looking for is always a VarRegion.

This was triggering assertion failures when analyzing the LLVM codebase.  This
is fallout from r175988.

I've got delta chewing away on a test case, but I wanted the fix to go
in now.