c++: Small pt.c cleanups

c++: Small pt.c cleanups

Now I know about VAR_OR_FUNCTION_DECL_P I found a place to use it.
Also positively checking for a function_decl is clearer than
negatively checking for things that are not.

	gcc/cp/
	* pt.c (primary_template_specialization_p): Use
	VAR_OR_FUNCTION_DECL_P.
	(tsubst_template_decl): Check for FUNCTION_DECL, not !TYPE && !VAR
	for registering a specialization.