Provide normalized and denormal format version of real_isdenormal.

Provide normalized and denormal format version of real_isdenormal.

Implement a variant of real_isdenormal() to be used within real.cc
where the argument is known to be in denormal format.  Rewrite
real_isdenormal() for use outside of real.cc where the argument is
known to be normalized.

gcc/ChangeLog:

	* real.cc (real_isdenormal): New.
	(encode_ieee_single): Call real_isdenormal.
	(encode_ieee_double): Same.
	(encode_ieee_extended): Same.
	(encode_ieee_quad): Same.
	(encode_ieee_half): Same.
	(encode_arm_bfloat_half): Same.
	* real.h (real_isdenormal): Add mode argument.  Rewrite for
	normalized values.
	* value-range.cc (frange::flush_denormals_to_zero): Pass mode to
	real_isdenormal.