[AtomicExpand][RISCV] Call shouldExpandAtomicRMWInIR before widenPartwordAtomicRMW (#80947)

This gives the target a chance to keep an atomicrmw op that is smaller
than the minimum cmpxchg size. This is needed to support the Zabha
extension for RISC-V which provides i8/i16 atomicrmw operations, but
does not provide an i8/i16 cmpxchg or LR/SC instructions.

This moves the widening until after the target requests
LLSC/CmpXChg/MaskedIntrinsic expansion. Once we widen, we call
shouldExpandAtomicRMWInIR again to give the target another chance to
make a decision about the widened operation.

I considered making the targets return AtomicExpansionKind::Expand or a
new expansion kind for And/Or/Xor, but that required the targets to
special case And/Or/Xor which they weren't currently doing.