[ast] Change printing of {Variable::maybe_assigned}.

This reverses the printing scheme for the flag in question to make it
print the "positive" analysis results (i.e. variable never assigned)
instead of the "negative" results (i.e. variable maybe assigned). This
helps to spot false-positives which are much more dangerous in practice
compared to missed optimization opportunities.

R=neis@chromium.org

Review-Url: https://codereview.chromium.org/2498353002
Cr-Commit-Position: refs/heads/master@{#40993}

src/ast/scopes.cc

@@ -1380,9 +1380,9 @@ static void PrintVar(int indent, Variable* var) {
       PrintF("forced context allocation");
       comma = true;
     }
-    if (var->maybe_assigned() == kMaybeAssigned) {
+    if (var->maybe_assigned() == kNotAssigned) {
       if (comma) PrintF(", ");
-      PrintF("maybe assigned");
+      PrintF("never assigned");
     }
     PrintF("\n");
   }