[RISCV] Refactor RISCV::hasAllWUsers to hasAllNBitUsers similar to RISCVISelDAGToDAG's version. NFC

Move to RISCVInstrInfo since we need RISCVSubtarget now.

Instead of asking if only the lower 32 bits are used we can now
ask if the lower N bits are used. This will be needed by a future
patch.