Reproduction:

using System.Numerics;
using System.Runtime.CompilerServices;

SideEffects(null, new Vector<int>[] { });

[MethodImpl(MethodImplOptions.NoInlining)]
static Vector<int> SideEffects(Vector<int>* a, Vector<int>[] b)
{
    var v = Vector.AndNot(*a, b[0]);

    return v;
}

Actual result:

> dotnet run
Unhandled exception. System.IndexOutOfRangeException: Index was outside the bounds of the array.

Expected result:

> dotnet run
Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.

This happens because of this code:

Which does not account for side effects. Important note: since the importation code does not propagate the side effect flags properly (yet), they cannot be relied on when determining whether it is safe to swap at that point.

Possible solutions:

1. Aggressively spill to temps - bad from CQ & TP standpoints.
2. Use GTF_REVERSE_OPS - bad from the standpoint that this flag should be removed.
3. Fix the operands up in LIR, where the tree walk no longer determines the execution order - probably cleanest but also most complex solution.

Note: the problematic code will become less centralized after #53450 is merged. See my review notes over there.