[RISCV] Use SUB instead of XOR in lowerShiftLeftParts/lowerShiftRightParts./

[RISCV] Use SUB instead of XOR in lowerShiftLeftParts/lowerShiftRightParts./

isel is now capable of turning the SUB into XOR for shift amounts.
Though it uses NOT instead of XOR with ShiftSize-1.

By using SUB during lowering we enable more DAG combines with
other arithmetic on the shift amount.