[RISCV] Teach RISCVDAGToDAGISel::selectShiftMask to replace sub from constant with neg.

If the shift amount is (sub C, X) where C is 0 modulo the size of
the shift, we can replace it with neg or negw.

Similar is is done for AArch64 and X86.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D119089