Swap the order of widening so that we widen to the next power-of-2 first when legalizing G_LOAD.
Also, provide a minimum type for the power of 2 to disallow s2 + s1. Clamping ought to disallow s2 and s1, but I think it's better to be explicit about the expected minimum size.
We probably need a similar change for G_STORE, but it seems to be a bit more finnicky. So, let's just handle G_LOAD for now.