[AArch64][SVE2] Generate XAR (#77160)

Bitwise exclusive OR and rotate right by immediate

Select xar (x, y, imm) for the following pattern:
    or (shl (xor x, y), nBits-imm), (shr (xor x, y), imm)

This is essentially:
    rotr (xor(x, y), imm)