Resolves #186527. The issue points out that GPRPair merge logic in the RISCVMoveMerger pass was being called on a target without `zdinx` or `experimental-P`, triggering an unreachable in `getGPRPairCopyOpcode`. This patch fixes this issue by guarding `isEvenRegisterCopy` and `isOddRegisterCopy` to return false when neither Zdinx nor P is present.