Improved RuntimeHelpers.ConvertLength codegen (#3608)

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## Follow up from #3520 
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## PR Type
What kind of change does this PR introduce?
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- Optimization
<!-- - Bugfix -->
<!-- - Feature -->
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
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## What is the current behavior?
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The codegen for the second branch in `RuntimeHelpers.ConvertLength` does a signed division:

https://github.com/windows-toolkit/WindowsCommunityToolkit/blob/9b75c9f910f999834c64bd00e522d7ae464c121b/Microsoft.Toolkit.HighPerformance/Helpers/Internals/RuntimeHelpers.cs#L43-L46

This is not the best for the codegen, as the JIT has to handle the sign in that division, resulting in the following:

```asm
; [System.Byte, System.Private.CoreLib],[System.Numerics.Vector4, System.Numerics.Vectors]
ConvertLength[TFrom, TTo](Int32)
    L0000: mov eax, ecx
    L0002: sar eax, 0x1f
    L0005: and eax, 0xf
    L0008: add eax, ecx
    L000a: sar eax, 4
    L000d: ret
```

## What is the new behavior?
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Avoided that with a cast to `uint`, since the length is guaranteed to be a positive value in `[0, int.MaxValue]` anyway:

```asm
; [System.Byte, System.Private.CoreLib],[System.Numerics.Vector4, System.Numerics.Vectors]
    L0000: mov eax, ecx
    L0002: shr eax, 4
    L0005: ret
```

Perfect! 😄🎉

## PR Checklist

Please check if your PR fulfills the following requirements:

- [X] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] ~~Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->~~
- [ ] ~~Sample in sample app has been added / updated (for bug fixes / features)~~
    - [ ] ~~Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)~~
- [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [X] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [X] Contains **NO** breaking changes

Author: msftbot[bot]

Microsoft.Toolkit.HighPerformance/Helpers/Internals/RuntimeHelpers.cs

@@ -42,7 +42,7 @@ public static unsafe int ConvertLength<TFrom, TTo>(int length)
             }
             else if (sizeof(TFrom) == 1)
             {
-                return length / sizeof(TTo);
+                return (int)((uint)length / (uint)sizeof(TTo));
             }
             else
             {