Only synthesize in the initial enqueuer trace.

Change-Id: Ib34e0a31e807ee8f1e5eaa5c6c37099903264d11

src/main/java/com/android/tools/r8/shaking/Enqueuer.java

diff --git a/src/main/java/com/android/tools/r8/shaking/Enqueuer.java b/src/main/java/com/android/tools/r8/shaking/Enqueuer.java
index a1333fb..6c7a4fe 100644
--- a/src/main/java/com/android/tools/r8/shaking/Enqueuer.java
+++ b/src/main/java/com/android/tools/r8/shaking/Enqueuer.java
@@ -2581,6 +2581,9 @@
   }
 
   private void synthesize() {
+    if (!mode.isInitialTreeShaking()) {
+      return;
+    }
     // First part of synthesis is to create and register all reachable synthetic additions.
     // In particular these additions are order independent, i.e., it does not matter which are
     // registered first and no dependencies may exist among them.