ARM64: print canonical syntax for add/sub (imm) instructions.

Since these instructions only accept a 12-bit immediate, possibly shifted left
by 12, the canonical syntax used by the architecture reference manual is "#N {,
lsl #12 }". We should accept an immediate that has already been shifted, (e.g.

Also, print a comment giving the full addend since it can be helpful.

llvm-svn: 207633