[RISCV] Have customLegalizeToWOp truncate to the original type instead of i32...

[RISCV] Have customLegalizeToWOp truncate to the original type instead of i32 now that we use it for i8/i16 as well.

239cfbcc add support for legalizing
i8/i16 UDIV/UREM/SDIV to use *W instructions. So we need to truncate
to i8/i16 if we're legalizing one of those.