[AArch64InstPrinter] Change printAddSubImm to comment imm value when shifted

Add a comment when there is a shifted value,
    add x9, x0, #291, lsl #12 ; =1191936
but not when the immediate value is unshifted,
    subs x9, x0, #256 ; =256
when the comment adds nothing additional to the reader.

Differential Revision: https://reviews.llvm.org/D107196