The types Vector64<T> and Vector128<T> are intended to map to ABI Short Vector fundamental types. These should be passed in vector registers.

Current code will pass these in integer registers.

IsShortVector can be crudely implemented as follows:

bool Compiler::IsShortVector(CORINFO_CLASS_HANDLE hClass)
{
    return isIntrinsicType(hClass) && (getBaseTypeOfSIMDType(hClass) != TYP_UNKNOWN);
}

Compiler::getPrimitiveTypeForStruct() can be trivially updated to return TYP_SIMD8 or TYP_SIMD16 when IsShortVector() == true

I made an initial naive attempt to implement and failed, but I really need a design discussion.

@CarolEidt @tannergooding @dotnet/arm64-contrib @dotnet/jit-contrib

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