[AArch64][GlobalISel] Fix a crash during selection of a G_ZEXT(s8 = G_LOAD)

We have special handling for a zext of a load <32b because the load does a zext
for free. In that case, we just select the G_ZEXT as if it were a copy but this
triggered the copy checking code to balk at the mismatched size.

This was being hidden because normally these get combined into G_ZEXTLOAD but
for atomics this doesn't happen. The test case here just uses a normal load
because the particular atomic isn't supported yet anyway.