[AArch64] Add store post-index update folding regression tests for the load/store optimizer.

Add regression tests for the following transformation:

  str X, [x20]
   ...
  add x20, x20, #32
   ->
  str X, [x20], #32

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209715 91177308-0d34-0410-b5e6-96231b3b80d8