[RISCV] Use _TIED form of VWADD(U)_WX/VWSUB(U)_WX to avoid early clobber.

One of the sources is the same size as the destination so that source
doesn't have an overlap with the destination register. By using the _TIED
form we avoid an early clobber contraint for that source.

This matches what was already done for instrinsics. ConvertToThreeAddress
will fix it if it can't stay tied.