[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210113 91177308-0d34-0410-b5e6-96231b3b80d8