[Support] Explicitly state that KnownBits::getMinValue/getMaxValue are UNSIGNED values. NFCI.

Update the comment to make this clear, following the same approach as APInt.

llvm/include/llvm/Support/KnownBits.h

@@ -113,13 +113,13 @@ public:
     Zero.setSignBit();
   }
 
-  /// Return the minimal value possible given these KnownBits.
+  /// Return the minimal unsigned value possible given these KnownBits.
   APInt getMinValue() const {
     // Assume that all bits that aren't known-ones are zeros.
     return One;
   }
 
-  /// Return the maximal value possible given these KnownBits.
+  /// Return the maximal unsigned value possible given these KnownBits.
   APInt getMaxValue() const {
     // Assume that all bits that aren't known-zeros are ones.
     return ~Zero;