[RISCV] Reuse lowerToScalableOp for more nodes. NFC (#151911)
A lot of fixed-length custom lowerings just involve inserting the
operands into a scalable container and extracting the result out, and
lowerToScalableOp already does this.
We just need to teach it to handle operands with different element types
(but same vector element count), and we can reuse it for
vselect/zext/sext/setcc/fcopysign.