Fixed bug and added tests for this

src/libraries/Common/tests/Tests/System/StringTests.cs

@@ -4699,6 +4699,8 @@ public static void Remove_Invalid()
         [InlineData("aaaAaaaaaaa", 'A', 'a', "aaaaaaaaaaa")]
         // Single iteration of vectorised path; 3 remainders handled by vectorized path
         [InlineData("AaaaaaaaaAa", 'A', 'a', "aaaaaaaaaaa")]
+        // Single iteration of vectorized path; 0 remainders handled by vectorized path
+        [InlineData("aaaaaaaaaAa", 'A', 'a', "aaaaaaaaaaa")]
         // Eight chars before a match (copyLength > 0), single iteration of vectorized path for the remainder
         [InlineData("12345678AAAAAAA", 'A', 'a', "12345678aaaaaaa")]
         // ------------------------- For Vector<ushort>.Count == 16 (AVX2) -------------------------
@@ -4707,6 +4709,8 @@ public static void Remove_Invalid()
         [InlineData("aaaAaaaaaaaAaaaaaaa", 'A', 'a', "aaaaaaaaaaaaaaaaaaa")]
         // Single iteration of vectorised path; 3 remainders handled by vectorized path
         [InlineData("AaaaaaaaAaaaaaaaaAa", 'A', 'a', "aaaaaaaaaaaaaaaaaaa")]
+        // Single iteration of vectorized path; 0 remainders handled by vectorized path
+        [InlineData("aaaaaaaaaaaaaaaaaAa", 'A', 'a', "aaaaaaaaaaaaaaaaaaa")]
         // Sixteen chars before a match (copyLength > 0), single iteration of vectorized path for the remainder
         [InlineData("1234567890123456AAAAAAAAAAAAAAA", 'A', 'a', "1234567890123456aaaaaaaaaaaaaaa")]
         // ----------------------------------- General test data -----------------------------------

src/libraries/System.Private.CoreLib/src/System/String.Manipulation.cs

@@ -1021,7 +1021,7 @@ public string Replace(char oldChar, char newChar)
 
                 nuint lengthToExamine = remainingLength - (nuint)Vector<ushort>.Count;
 
-                if (lengthToExamine > 0)
+                if ((nint)lengthToExamine > 0)
                 {
                     do
                     {
@@ -1043,7 +1043,7 @@ public string Replace(char oldChar, char newChar)
                 // We perform this operation even if there are 0 elements remaining, as it is cheaper than the
                 // additional check which would introduce a branch here.
 
-                i = (nuint)((uint)Length - Vector<ushort>.Count);
+                i = (uint)(Length - Vector<ushort>.Count);
                 original = Vector.LoadUnsafe(ref GetRawStringDataAsUInt16(), i);
                 equals = Vector.Equals(original, oldChars);
                 results = Vector.ConditionalSelect(equals, newChars, original);