[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.

llvm-svn: 209711