frange: flush denormals to zero

frange: flush denormals to zero

For some architectures (or for -funsafe-math-optimizations) we may
flush denormals to zero, in which case we need to be careful to
extend the ranges to the appropriate zero.  This patch does exactly that.
For a range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x]
we flush to [+0.0, x].

gcc/ChangeLog:

	* value-range.cc (frange::flush_denormals_to_zero): New.
	(frange::set): Call flush_denormals_to_zero.
	* value-range.h (class frange): Add flush_denormals_to_zero.