[NFC] Update a comment about synchronization in GUFA (#7312)

The comment mentioned that in principle we might be able to optimize
atomic struct.gets when we know what the stored value we are loading
will be. This is not correct because the write and the read would still
create a synchronization edge in that case. Update the comment
accordingly.

Author: tlively

src/passes/GUFA.cpp

@@ -153,10 +153,7 @@ struct GUFAOptimizer
     if (Properties::getMemoryOrder(curr) != MemoryOrder::Unordered) {
       // This load might synchronize with some store, and if we replaced the
       // load with a constant or with a load from a global, it would not
-      // synchronize with that store anymore. Since we know what value the store
-      // must write, and we know it is the same as every other store to the same
-      // location, it's possible that optimizing here would be allowable, but
-      // for now be conservative and do not optimize.
+      // synchronize with that store anymore.
       return;
     }
 

test/lit/passes/gufa-refs.wast

@@ -6171,19 +6171,22 @@
     )
     (drop
       ;; This is optimizable. It reads from shared memory, but there is only one
-      ;; possible value that can be read.
+      ;; possible value that can be read and it is not atomic, so does not form
+      ;; a synchronization edge.
       (struct.get $A 0
         (local.get 0)
       )
     )
     (drop
-      ;; We do not (yet) optimize atomic gets.
+      ;; We do not optimize atomic gets, since they might synchronize with a
+      ;; write.
       (struct.atomic.get acqrel $A 0
         (local.get 0)
       )
     )
     (drop
-      ;; We do not (yet) optimize atomic gets.
+      ;; We do not optimize atomic gets, since they might synchronize with a
+      ;; write.
       (struct.atomic.get $A 0
         (local.get 0)
       )