This patch performs a widening transformation of bitwise atomicrmw or, xor, and and applies it prior to tryExpandAtomicRMW. This operates similarly to convertCmpXchgToIntegerType. For these operations, the i8/i16 atomicrmw can be implemented in terms of the 32-bit atomicrmw by appropriately manipulating the operands. There is no functional change for the handling of partword or/xor, but the transformation for partword and is new.
The advantage of performing this transformation early is that the same code-path can be used regardless of the approach used to expand the atomicrmw (AtomicExpansionKind). i.e. the same logic is used for AtomicExpansionKind::CmpXchg and can also be used by the intrinsic-based expansion in D47882.