[clang] [llvm] [LoongArch] Support amcas[_db].{b/h/w/d} instructions. (PR #114189)

[WÁNG Xuěruì via cfe-commits]

https://github.com/xen0n commented:

I think this is getting more convoluted than necessary? For example, I didn't see a `setOperationAction(ISD::ATOMIC_CMP_SWAP, Legal)` call gated on presence of the `lamcas` feature, which I think would render a lot (if not most) of the changes here made to the various `shouldXxx` predicates unnecessary.

https://github.com/llvm/llvm-project/pull/114189