[RISCV] RISCVDAGToDAGISel::selectShiftMask to shift by (sub size-1, X).

If the shift amount is (sub C, X) where C is -1 modulo the size of
the shift, we can replace the sub with a NOT.

We could also use XORI X, size-1, but NOT would work better with
c.not from the future Zce extension.