[AArch64] Add some more regression tests for store pre-index update folding in the load/store optimizer.

Add tests for the following transform:

 add x8, x8, #16
  ...
 str X, [x8]
  ->
 str X, [x8, #16]!

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210021 91177308-0d34-0410-b5e6-96231b3b80d8