Simplify logic for handling negative values

src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs

@@ -3481,38 +3481,20 @@ public static BigInteger TrailingZeroCount(BigInteger value)
 
             ulong result = 0;
 
-            if (value._sign >= 0)
-            {
-                // When the value is positive, we simply need to do a tzcnt for all bits until we find one set
-
-                uint part = value._bits[0];
+            // As explained in https://github.com/dotnet/runtime/issues/77720, both positive values and
+            // their two's-compliment negative representation will share the same TrailingZeroCount,
+            // so the stored sign of value does not matter
 
-                for (int i = 1; (part == 0) && (i < value._bits.Length); i++)
-                {
-                    part = value._bits[i];
-                    result += (sizeof(uint) * 8);
-                }
+            uint part = value._bits[0];
 
-                result += uint.TrailingZeroCount(part);
-            }
-            else
+            for (int i = 1; (part == 0) && (i < value._bits.Length); i++)
             {
-                // When the value is negative, we need to tzcnt the two's complement representation
-                // We'll do this "inline" to avoid needing to unnecessarily allocate.
-
-                uint part = ~value._bits[0] + 1;
-
-                for (int i = 1; (part == 0) && (i < value._bits.Length); i++)
-                {
-                    // Simply process bits, adding the carry while the previous value is zero
-
-                    part = ~value._bits[i] + 1;
-                    result += (sizeof(uint) * 8);
-                }
-
-                result += uint.TrailingZeroCount(part);
+                part = value._bits[i];
+                result += (sizeof(uint) * 8);
             }
 
+            result += uint.TrailingZeroCount(part);
+
             return result;
         }