Do not use cast for finding const-number instruction

Bug: 175132220
Change-Id: I9c0e227ac6c35272f1e415443f1855deb7cf51da

src/main/java/com/android/tools/r8/ir/code/ConstNumber.java

diff --git a/src/main/java/com/android/tools/r8/ir/code/ConstNumber.java b/src/main/java/com/android/tools/r8/ir/code/ConstNumber.java
index 763d520..b849f75 100644
--- a/src/main/java/com/android/tools/r8/ir/code/ConstNumber.java
+++ b/src/main/java/com/android/tools/r8/ir/code/ConstNumber.java
@@ -47,7 +47,10 @@
   }
 
   public static ConstNumber asConstNumberOrNull(Instruction instruction) {
-    return (ConstNumber) instruction;
+    if (instruction == null) {
+      return null;
+    }
+    return instruction.asConstNumber();
   }
 
   @Override