[RISCV] Improve codegen for i8/i16 'atomicrmw xchg a, {0,-1}'

As noted in <https://github.com/llvm/llvm-project/issues/64090>, it's
more efficient to lower a partword 'atomicrmw xchg a, 0` to and amoand
with appropriate mask. There are a range of possible ways to go about
this - e.g. writing a combine based on the
`llvm.riscv.masked.atomicrmw.xchg` intrinsic, or introducing a new
interface to AtomicExpandPass to allow target-specific atomics
conversions, or trying to lift the conversion into AtomicExpandPass
itself based on querying some target hook. Ultimately I've gone with
what appears to be the simplest approach - just covering this case in
emitMaskedAtomicRMWIntrinsic. I perhaps should have given that hook a
different name way back when it was introduced.

This also handles the `atomicrmw xchg a, -1` case suggested by Craig
during review.

Fixes https://github.com/llvm/llvm-project/issues/64090

Differential Revision: https://reviews.llvm.org/D156801