Provide a default range_operator via range_op_handler.

Provide a default range_operator via range_op_handler.

range_op_handler now provides a default range_operator for any opcode,
so there is no longer a need to check for a valid operator.

	* gimple-range-op.cc (gimple_range_op_handler): Set m_operator
	manually as there is no access to the default operator.
	(cfn_copysign::fold_range): Don't check for validity.
	(cfn_ubsan::fold_range): Ditto.
	(gimple_range_op_handler::maybe_builtin_call): Don't set to NULL.
	* range-op.cc (default_operator): New.
	(range_op_handler::range_op_handler): Use default_operator
	instead of NULL.
	(range_op_handler::operator bool): Move from header, compare
	against default operator.
	(range_op_handler::range_op): New.
	* range-op.h (range_op_handler::operator bool): Move.