Avoid MSVC's C6282 warning (assignment of constant in Boolean context).

By courtesy of /analyze... :-}

Review URL: https://codereview.chromium.org/846223002

Cr-Commit-Position: refs/heads/master@{#26032}

src/ast.h

@@ -3157,7 +3157,8 @@ class AstVisitor BASE_EMBEDDED {
     if (stack_overflow_) return true;                       \
     StackLimitCheck check(zone_->isolate());                \
     if (!check.HasOverflowed()) return false;               \
-    return (stack_overflow_ = true);                        \
+    stack_overflow_ = true;                                 \
+    return true;                                            \
   }                                                         \
                                                             \
  private:                                                   \