[AArch64] AArch64LoadStoreOptimizer: fix bug in pre-inc check iterator

Summary:
Fix case where a pre-inc/dec load/store would not be formed if the
add/sub that forms the inc/dec part of the operation was the first
instruction in the block being examined.

Reviewers: mcrosier, jmolloy, t.p.northover, junbuml

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D16785

llvm-svn: 260275