tree-cfg: check placement of returns_twice calls

tree-cfg: check placement of returns_twice calls

When a returns_twice call has an associated abnormal edge, the edge
corresponds to the "second return" from the call. It wouldn't make sense
if any executable statements appeared between the call and the
destination of the edge (they wouldn't be re-executed upon the "second
return"), so verify that.

gcc/ChangeLog:

	* tree-cfg.cc (gimple_verify_flow_info): Check placement of
	returns_twice calls.