[AArch64][GlobalISel] Improve legalization for odd-type G_LOAD

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.

Differential Revision: https://reviews.llvm.org/D107013