compiler: Use backend interface for binary expressions.

	* go-gcc.cc (Gcc_backend::conditional_expression): Add btype
	parameter.
	(operator_to_tree_code): New static function.
	(Gcc_backend::binary_expression): New function.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@206696 138bc75d-0d04-0410-961f-82ee72b054a4