Improve code generation for ARM64 VisitArrayGet/Set.

We prefer the code sequence
   add temp, obj, #offset
   ldr out, [temp, index LSL #shift_amount]
to
   add temp, obj, index LSL #shift_amount
   ldr out, [temp, #offset]

Change-Id: I98f51a1b5a5ecd84c677d6dbd4c4bfc0f157f5e2