Add a static method GroupedList#numElements to allow to count the number of deps from a compressed GroupedList without uncompressing it. Also some minor GC improvements.

PiperOrigin-RevId: 177338852
diff --git a/src/main/java/com/google/devtools/build/skyframe/InMemoryNodeEntry.java b/src/main/java/com/google/devtools/build/skyframe/InMemoryNodeEntry.java
index ee80c29..2ad10d2 100644
--- a/src/main/java/com/google/devtools/build/skyframe/InMemoryNodeEntry.java
+++ b/src/main/java/com/google/devtools/build/skyframe/InMemoryNodeEntry.java
@@ -233,6 +233,11 @@
     return GroupedList.create(directDeps);
   }
 
+  public int getNumDirectDeps() {
+    Preconditions.checkState(isDone(), "no deps until done. NodeEntry: %s", this);
+    return GroupedList.numElements(directDeps);
+  }
+
   @Override
   @Nullable
   public synchronized ErrorInfo getErrorInfo() {