[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.

llvm-svn: 222333