Implement some miscellaneous zero accessors for Value_Range.

This adds some missing accessors to the type agnostic Value_Range
class.  They'll be used in the upcoming IPA work.

gcc/ChangeLog:

	* value-range.h (class Value_Range): Implement set_zero,
	set_nonzero, and nonzero_p.

gcc/value-range.h

@@ -497,6 +497,9 @@ class Value_Range
   bool contains_p (tree cst) const { return m_vrange->contains_p (cst); }
   bool singleton_p (tree *result = NULL) const
     { return m_vrange->singleton_p (result); }
+  void set_zero (tree type) { return m_vrange->set_zero (type); }
+  void set_nonzero (tree type) { return m_vrange->set_nonzero (type); }
+  bool nonzero_p () const { return m_vrange->nonzero_p (); }
   bool zero_p () const { return m_vrange->zero_p (); }
   wide_int lower_bound () const; // For irange/prange comparability.
   wide_int upper_bound () const; // For irange/prange comparability.