[frange] Remove redundant known_isnan() checks.

The known_isnan() method is a subset of maybe_isnan().  This patch
removes redundant calls to known_isnan().

gcc/ChangeLog:

	* range-op-float.cc (operator_lt::op1_range): Remove known_isnan check.
	(operator_lt::op2_range): Same.
	(operator_le::op1_range): Same.
	(operator_le::op2_range): Same.
	(operator_gt::op1_range): Same.
	(operator_gt::op2_range): Same.
	(operator_ge::op1_range): Same.
	(operator_ge::op2_range): Same.
	(foperator_unordered_lt::op1_range): Same.
	(foperator_unordered_lt::op2_range): Same.
	(foperator_unordered_le::op1_range): Same.
	(foperator_unordered_le::op2_range): Same.
	(foperator_unordered_gt::op1_range): Same.
	(foperator_unordered_gt::op2_range): Same.
	(foperator_unordered_ge::op1_range): Same.
	(foperator_unordered_ge::op2_range): Same.