[AArch64][GlobalISel] Ensure atomic loads always get assigned GPR destinations

The default register bank selection code for G_LOAD assumes that we ought to
use a FPR when the load is casted to a float/double.

For atomics, this isn't true; we should always use GPRs.

Without this patch, we crash in the following example:

https://godbolt.org/z/MThjas441

Also make the code a little more stylistically consistent while we're here.

Also test some other weird cast combinations as well.

Differential Revision: https://reviews.llvm.org/D109771