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

Add regression tests for the following transformation:

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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209711 91177308-0d34-0410-b5e6-96231b3b80d8