Add helpers to test whether an optab can be implemented

Add helpers to test whether an optab can be implemented

The vectoriser and vector lowering passes both had tests of the form:

    if (op
	&& (optab_handler (op, compute_mode) != CODE_FOR_nothing
	    || optab_libfunc (op, compute_mode)))
      ...success...
    if (code == MULT_HIGHPART_EXPR
	&& can_mult_highpart_p (compute_mode,
				TYPE_UNSIGNED (compute_type)))
      ...success...

This patch adds helper routines for this kind of test, so that it's
easier to handle other optab alternatives in a similar way.

gcc/
	* optabs-query.cc (can_open_code_p, can_implement_p): Declare.
	* optabs-query.h (can_open_code_p, can_implement_p): New functions.
	* optabs-tree.cc (target_supports_op_p): Use can_implement_p.
	* tree-vect-stmts.cc (vectorizable_operation): Likewise.
	* tree-vect-generic.cc (get_compute_type): Likewise.  Remove code
	parameter.
	(expand_vector_scalar_condition, expand_vector_conversion)
	(expand_vector_operations_1): Update calls accordingly.