[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

llvm/lib/Target/AArch64/AArch64Subtarget.h

@@ -115,7 +115,7 @@ public:
   bool isCortexA57() const { return CPUString == "cortex-a57"; }
   bool isCortexA53() const { return CPUString == "cortex-a53"; }
 
-  bool useAA() const override { return isCortexA53() || isCortexA57(); }
+  bool useAA() const override { return isCortexA53(); }
 
   /// getMaxInlineSizeThreshold - Returns the maximum memset / memcpy size
   /// that still makes it profitable to inline the call.