[AArch64] Add regression tests for the load/store optimizer which cover post-index update folding with sub rather than add.

The tests check that the following transform happens:

  (ldr|str) X, [x20]
   ...
  sub x20, x20, #16
   ->
  (ldr|str) X, [x20], #-16

with X being either w0, x0, s0, d0 or q0.