Fix assume inserter

If we update the dynamic type based on both null and non null we hit assertions later on

Change-Id: If03a63aa0f0aef20fa0129bccaa051d5780a782c
diff --git a/src/main/java/com/android/tools/r8/ir/optimize/AssumeInserter.java b/src/main/java/com/android/tools/r8/ir/optimize/AssumeInserter.java
index fc63ec3..8d6c10c 100644
--- a/src/main/java/com/android/tools/r8/ir/optimize/AssumeInserter.java
+++ b/src/main/java/com/android/tools/r8/ir/optimize/AssumeInserter.java
@@ -424,7 +424,7 @@
             return false;
           }
 
-          if (!otherAssumedValueInfo.isNonNull()) {
+          if (!otherAssumedValueInfo.isNonNull() && !otherAssumedValueInfo.isNull()) {
             // This is not redundant, but we can strengthen it with the dynamic type information
             // from the other assume instruction.
             assumedValueInfo.setDynamicType(