[llvm] [RISCV][MC] Restrict mips.ldp / mips.sdp to RV64 only (PR #227043)

[Djordje Todorovic via llvm-commits]

djtodoro wrote:

> Thanks for reviewing!
> 
> > I would additional guard into the RISCVSubtarget::useLoadStorePairs() predicate for this as well.
> 
> The [load/store optimizer](https://github.com/llvm/llvm-project/blob/ce0affc054bbdae90c7c7d8a50bfabd5047ebb50/llvm/lib/Target/RISCV/RISCVLoadStoreOptimizer.cpp#L489-L507) only forms `MIPS_LDP/MIPS_SDP` from the RV64-only `LD/SD` opcodes. Adding an RV64 guard to this predicate would also disable `LWP/SWP` pairing on RV32, so I think we should keep it unchanged.

Oh, it does make sense. LGTM!

https://github.com/llvm/llvm-project/pull/227043