Also use monomorphic method states for interface methods

Change-Id: I983cb0222a8321766d456afb34f57474833b09f2
diff --git a/src/main/java/com/android/tools/r8/optimize/argumentpropagation/codescanner/VirtualRootMethodsAnalysisBase.java b/src/main/java/com/android/tools/r8/optimize/argumentpropagation/codescanner/VirtualRootMethodsAnalysisBase.java
index 6130e36..3343089 100644
--- a/src/main/java/com/android/tools/r8/optimize/argumentpropagation/codescanner/VirtualRootMethodsAnalysisBase.java
+++ b/src/main/java/com/android/tools/r8/optimize/argumentpropagation/codescanner/VirtualRootMethodsAnalysisBase.java
@@ -226,15 +226,8 @@
             VirtualRootMethod singleDispatchTarget = virtualRootMethod.getSingleDispatchTarget();
             if (singleDispatchTarget != null) {
               virtualRootMethod.forEach(
-                  method -> {
-                    // Interface methods can have siblings and can therefore not be mapped to their
-                    // unique non-abstract implementation, unless the interface method does not have
-                    // any siblings.
-                    setRootMethod(method, virtualRootMethod, singleDispatchTarget);
-                  });
-              if (!singleDispatchTarget.getMethod().getHolder().isInterface()) {
-                monomorphicVirtualNonRootMethods.add(singleDispatchTarget.getMethod());
-              }
+                  method -> setRootMethod(method, virtualRootMethod, singleDispatchTarget));
+              monomorphicVirtualNonRootMethods.add(singleDispatchTarget.getMethod());
             } else {
               virtualRootMethod.forEach(
                   method -> setRootMethod(method, virtualRootMethod, virtualRootMethod));