[CFE] Fix bug in benchmarker where it used the last comparison (and not the combined) to signal if anything was significant

It only resulted in it saying "no change" after it had printed changes,
but still.

Change-Id: I198265fe89bdb473eff3a2725e2a17323f560fbd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/449800
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Jens Johansen <[email protected]>

Author: jensjoha

pkg/front_end/tool/benchmarker.dart

@@ -236,7 +236,7 @@ bool _compare(List<Map<String, num>> from, List<Map<String, num>> to) {
       }
     }
     if (fromForCaption.isEmpty || toForCaption.isEmpty) continue;
-    somethingWasSignificant = _compareSingle(
+    somethingWasSignificant |= _compareSingle(
       toForCaption,
       fromForCaption,
       caption,