[RISCV] Support vwsll in combineBinOp_VLToVWBinOp_VL (#87620)

If the subtarget has +zvbb then we can attempt folding shl and shl_vl to
vwsll nodes.

There are few test cases where we still don't pick up the vwsll:
- For fixed vector vwsll.vi on RV32, see the FIXME for VMV_V_X_VL in
fillUpExtensionSupport for support implicit sign extension
- For scalable vector vwsll.vi we need to support ISD::SPLAT_VECTOR, see
#87249