[AArch64] Add a regression test for the load store optimizer.

We have a couple of regression tests for load/store pairing, but (to my knowledge) there are no regression tests for the load/store + add/sub folding.

As a first step towards increased test coverage of this area, this commit adds a test for one instance of a load + add to pre-indexed load transformation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209618 91177308-0d34-0410-b5e6-96231b3b80d8