Aarch64: elide barriers on all volatile operations

Andrew Haley said:

I notice that this works for simple volatile access, but it does not
 work for read-modify write operations such as, say,
 VarHandle.getAndAdd(), where the barrier instructions are not elided:

  ;; membar_release
   0x000003ffa8b049b8: dmb ish
   0x000003ffa8b049bc: add x10, x19, #0xc
   0x000003ffa8b049c0: ldxr wscratch2, [x10]
   0x000003ffa8b049c4: add wscratch1, wscratch2, #0x1
   0x000003ffa8b049c8: stxr wscratch2, wscratch1, [x10]
   0x000003ffa8b049cc: cbnz wscratch2, 0x000003ffa8b049c0
   0x000003ffa8b049d0: sub w10, wscratch1, #0x1
  ;; membar_acquire
   0x000003ffa8b049d4: dmb ishld ;*invokevirtual getAndAddInt {reexecute=0 rethrow=0 return_oop=0}
                                                             ; - java.lang.invoke.VarHandleInts$FieldInstanceReadWrite::getAndAdd@19 (line 206)
                                                             ; - java.lang.invoke.VarHandleGuards::guard_LI_I@33 (line 129)
                                                             ; - Sync::run@42 (line 35)

 I think we need to be consistent about how we handle these operations.