[C11-atomic] new test: limit precomputing values across acquire barriers

[Aldy Hernandez]

This is a similar test to the previous acquire test.  Here we are 
incorrectly caching 'x' and failing to reload it after the __ATOMIC_ACQUIRE.

+  i = x + y;
+
+  if (__atomic_load_n (&flag, __ATOMIC_ACQUIRE))
+    {
+      /* x here should not be reused from above.  */
+      k = x;
+    }

Note that there is technically a data race on the load of x+y.  See the 
explanation on my previous testcase.

OK for branch?
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