[ARM64] Fix wrong comment in load/store optimization pass.

ldr x1, [x0, #64]
add x0, x0, #64
 ->
ldr x1, [x0], #64

is not a valid transformation, the correct transformation (and what the code actually does) is:

ldr x1, [x0, #64]
add x0, x0, #64
 ->
ldr x1, [x0, #64]!

llvm-svn: 208998