Declare dconstm0 to go along with dconst0 and friends.

Declare dconstm0 to go along with dconst0 and friends.

Negating dconst0 is getting pretty old, and we will keep adding copies
of the same idiom.  Fixed by adding a dconstm0 constant to go along
with dconst1, dconstm1, etc.

gcc/ChangeLog:

	* emit-rtl.cc (init_emit_once): Initialize dconstm0.
	* gimple-range-op.cc (class cfn_signbit): Remove dconstm0
	declaration.
	* range-op-float.cc (zero_range): Use dconstm0.
	(zero_to_inf_range): Same.
	* real.h (dconstm0): New.
	* value-range.cc (frange::flush_denormals_to_zero): Use dconstm0.
	(frange::set_zero): Do not declare dconstm0.