c++: Avoid unnecessary empty class copy [94175].

c++: Avoid unnecessary empty class copy [94175].

A simple empty class copy is still simple when wrapped in a TARGET_EXPR, so
we need to strip that as well.  This change also exposed some unnecessary
copies in return statements, which when returning by invisible reference led
to <RETURN_EXPR <MEM_REF <RESULT_DECL>>>, which gimplify_return_expr didn't
like.  So we also need to strip the _REF when we eliminate the INIT_EXPR.

gcc/cp/ChangeLog
2020-03-19  Jason Merrill  <jason@redhat.com>

	PR c++/94175
	* cp-gimplify.c (simple_empty_class_p): Look through
	SIMPLE_TARGET_EXPR_P.
	(cp_gimplify_expr) [MODIFY_EXPR]: Likewise.
	[RETURN_EXPR]: Avoid producing 'return *retval;'.
	* call.c (build_call_a): Strip TARGET_EXPR from empty class arg.
	* cp-tree.h (SIMPLE_TARGET_EXPR_P): Check that TARGET_EXPR_INITIAL
	is non-null.