x64 Windows does not correctly handle __m128 returns.

[tannergooding]

The default calling convention for x64 Windows specifies that __m128, __m128i, and __m128d are returned in XMM0: https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention?view=vs-2019#return-values

These types correspond to the System.Runtime.Intrinsics.Vector128<T> type on the managed side and it is not currently being returned in XMM0.

dotnet/coreclr#23899 adds support for passing Vector128<T> across interop boundaries and so this will need to be correctly handled.

[tannergooding]

CC. @CarolEidt

[CarolEidt]

How is this distinct from #9578?

[tannergooding]

I forgot I logged the former and couldn't find it in GitHub search. I'll update the former with this specific scenario shortly and then will close this one.