[AArch64] Prevent spilling between ldxr/stxr pairs

Apply the same logic used to check if CMPXCHG nodes should be expanded
at -O0: the register allocator may end up spilling some register in
between the atomic load/store pairs, breaking the atomicity and possibly
stalling the execution.

Fixes PR48017

Reviewed By: efriedman

Differential Revision: https://reviews.llvm.org/D101163