Do not specify NAN sign in frange::set_nonnegative.

Do not specify NAN sign in frange::set_nonnegative.

After further reading of the IEEE 754 standard, it has become clear
that there are no guarantees with regards to the sign of a NAN when it
comes to any operation other than copy, copysign, abs, and negate.

Currently, set_nonnegative() is only used in one place in ranger
applicable to floating point values, when expanding unknown calls.
Since we already specially handle copy, copysign, abs, and negate, all
the calls to set_nonnegative() must be NAN-sign agnostic.

The cleanest solution is to leave the sign unspecificied in
frange::set_nonnegative().  Any special case, must be handled by the
caller.

gcc/ChangeLog:

	* value-range.cc (frange::set_nonnegative): Remove NAN sign handling.
	(range_tests_signed_zeros): Adjust test.