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

[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.

llvm-svn: 210021