[AArch64] Disable useAA for Cortex-A57.
Using AA during CodeGen is very useful for in-order cores. It is less useful for ooo cores. Also I find
enabling useAA for Cortex-A57 may generate worse code for some test cases. If useAA in codegen is improved 
and benefical for ooo cores, we can enable it again.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222333 91177308-0d34-0410-b5e6-96231b3b80d8