[analyzer] Use cast<> instead of getAs<> for a CFGElement known to be a CFGStmt.

When adding the next statement to the CoreEngine's work list, we take care
of all the special cases first. We certainly shouldn't be building
PostStmts with null statements (the diagnostics machinery assumes such
StmtPoints do not exist), and we should find out sooner if we're missing
a special case.

A refinement of r163402 that should help prevent further issues like PR13760.

llvm-svn: 163409