[CodeGen][ARM] Implement atomicrmw as pseudo operations at -O0

atomicrmw instructions are expanded by AtomicExpandPass before register allocation
into cmpxchg loops. Register allocation can insert spills between the exclusive loads
and stores, which invalidates the exclusive monitor and can lead to infinite loops.

To avoid this, reimplement atomicrmw operations as pseudo-instructions and expand them
after register allocation.

Floating point legalisation:
f16 ATOMIC_LOAD_FADD(*f16, f16) is legalised to
f32 ATOMIC_LOAD_FADD(*i16, f32) and then eventually
f32 ATOMIC_LOAD_FADD_16(*i16, f32)

Differential Revision: https://reviews.llvm.org/D101164