[Mono] Restore old code to solve the recent SpanHelpers regressions (#75917)

* bring back the old code...

* bring back more old code

* Use an ifdef around clr code instead of a separate file

* Delete SpanHelpers.Clr.cs

* Remove a remaining INumber<T> helper from mono

Co-authored-by: Jeff Handley <[email protected]>

Author: adamsitnik

src/libraries/System.Private.CoreLib/src/System/MemoryExtensions.cs

@@ -1617,6 +1617,7 @@ ref Unsafe.As<T, byte>(ref MemoryMarshal.GetReference(span)),
                                 Unsafe.Add(ref valueRef, 2),
                                 span.Length);
 
+#if !MONO // We don't have a mono overload for 4 values
                         case 4:
                             return SpanHelpers.LastIndexOfAnyValueType(
                                 ref spanRef,
@@ -1625,6 +1626,7 @@ ref Unsafe.As<T, byte>(ref MemoryMarshal.GetReference(span)),
                                 Unsafe.Add(ref valueRef, 2),
                                 Unsafe.Add(ref valueRef, 3),
                                 span.Length);
+#endif
 
                         default:
                             return LastIndexOfAnyProbabilistic(ref Unsafe.As<short, char>(ref spanRef), span.Length, ref Unsafe.As<short, char>(ref valueRef), values.Length);

src/libraries/System.Private.CoreLib/src/System/SpanHelpers.T.cs

@@ -2504,6 +2504,7 @@ private static int LastIndexOfAnyValueType<TValue, TNegator>(ref TValue searchSp
             return -1;
         }
 
+#if !MONO
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static int LastIndexOfAnyValueType<T>(ref T searchSpace, T value0, T value1, T value2, T value3, int length) where T : struct, INumber<T>
             => LastIndexOfAnyValueType<T, DontNegate<T>>(ref searchSpace, value0, value1, value2, value3, length);