[ValueTracking] Clarify comments on CannotBeOrderedLessThanZero and SignBitMustBeZero.

[arsenm]

LGTM

llvm/trunk/include/llvm/Analysis/ValueTracking.h

template <typename T> class ArrayRef;
   Intrinsic::ID getIntrinsicForCallSite(ImmutableCallSite ICS,
                                         const TargetLibraryInfo *TLI);
 
   /// Return true if we can prove that the specified FP value is never equal to
   /// -0.0.
   bool CannotBeNegativeZero(const Value *V, const TargetLibraryInfo *TLI,
                             unsigned Depth = 0);
 
-  /// Return true if we can prove that the specified FP value is either a NaN or
-  /// never less than 0.0.
-  /// If \p IncludeNeg0 is false, -0.0 is considered less than 0.0.
+  /// Return true if we can prove that the specified FP value is either NaN or
+  /// never less than -0.0.
+  ///
+  ///      NaN --> true
+  ///       +0 --> true
+  ///       -0 --> true
+  ///   x > +0 --> true
+  ///   x < -0 --> false
+  ///
   bool CannotBeOrderedLessThanZero(const Value *V, const TargetLibraryInfo *TLI);
 
-  /// \returns true if we can prove that the specified FP value has a 0 sign
-  /// bit.
+  /// Return true if we can prove that the specified FP value's sign bit is 0.
+  ///
+  ///      NaN --> true/false (depending on the NaN's sign bit)
+  ///       +0 --> true
+  ///       -0 --> false
+  ///   x > +0 --> true
+  ///   x < -0 --> false
+  ///
   bool SignBitMustBeZero(const Value *V, const TargetLibraryInfo *TLI);
 
   /// If the specified value can be set by repeating the same byte in memory,
   /// return the i8 value that it is represented with. This is true for all i8
   /// values obviously, but is also true for i32 0, i32 -1, i16 0xF0F0, double
   /// 0.0 etc. If the value can't be handled with a repeated byte store (e.g.
   /// i16 0x1234), return null.
   Value *isBytewiseValue(Value *V);