[RISCV] Implement codegen for cmpxchg on RV32IA

[RISCV] Implement codegen for cmpxchg on RV32IA

Utilise a similar ('late') lowering strategy to D47882. The changes to
AtomicExpandPass allow this strategy to be utilised by other targets which
implement shouldExpandAtomicCmpXchgInIR.

All cmpxchg are lowered as 'strong' currently and failure ordering is ignored.
This is conservative but correct.

Differential Revision: https://reviews.llvm.org/D48131

llvm-svn: 347914