[AArch64ISelDAGToDAG] Fix ORRWrs/ORRXrs usefulbits calculation bug

For the following case:

    t8: i32 = or t7, t4
    t10: i32 = ORRWrs t8, t8, TargetConstant:i32<73>

Current code wrongly returns (t8 >> shiftConstant) as the
UsefulBits of t8, which in fact is (t8 | (t8 >> shiftConstant)).

Reviewed by: sdesmalen, mdchen

Differential Revision: https://reviews.llvm.org/D102759