c++, coroutines: Use finish_if_stmt in a missed case.

Just shorter code.

gcc/cp/ChangeLog:

	* coroutines.cc
	(cp_coroutine_transform::wrap_original_function_body): Use
	finish_if_stmt instead of manually applying the same process.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>