JIT importation asserts with EnableAVX2=0

[dhartglassMSFT]

Bad importation for IsNegative and IsPositive for FP types

With DOTNET_EnableAVX2=0, this hits Assertion failed 'comp->compIsaSupportedDebugOnly(InstructionSet_AVX2)' in 'TestClass:Method1():System.Runtime.Intrinsics.Vector2561[double]' during 'Importation`:

    [MethodImpl(MethodImplOptions.NoInlining)]
    public static Vector256<double> Method1()
    {
        return Vector256.IsNegative(s_v256_double);
    }

This gets Assertion failed '(simdSize == 16) || ((simdSize == 32) && compOpportunisticallyDependsOn(InstructionSet_AVX2))' during 'Importation':

    [MethodImpl(MethodImplOptions.NoInlining)]
    public static Vector256<float> Method81()
    {
        return Vector256.IsPositive(s_v256_float);
    }

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