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

[Djordje Todorovic via llvm-commits]

djtodoro wrote:

This looks good to me.

> Do we need to be careful in the load/store optimisation pass too?

I would additional guard into the `RISCVSubtarget::useLoadStorePairs()` predicate for this as well.

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