analyzer: use switch in exploded_node::on_stmt

analyzer: use switch in exploded_node::on_stmt

This patch replaces a sequence of dyn_cast to different gimple stmt
types in exploded_node::on_stmt with a switch on the gimple_code.  This
makes clearer which kinds of stmt are currently treated as no-ops, as a
precursor to handling them properly.

No functional change intended.

gcc/analyzer/ChangeLog:
	* engine.cc (exploded_node::on_stmt): Replace sequence of dyn_cast
	with switch.