[release/7.0-rc2] [Mono] Restore old code to solve the recent SpanHel…

…pers regressions (#75996)

* 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: Adam Sitnik <adam.sitnik@gmail.com>
Co-authored-by: Jeff Handley <jeffhandley@users.noreply.github.com>

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -2457,4 +2457,7 @@
  <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnaryPlusOperators.cs" />
  <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnsignedNumber.cs" />
  </ItemGroup>
+ <ItemGroup Condition="'$(FeatureMono)' == 'true'">
+ <Compile Include="$(MSBuildThisFileDirectory)System\SpanHelpers.Mono.cs" />
+ </ItemGroup>
 </Project>

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);