Fix crash in assertion introduced by previous CL

Bug: 467374229
Change-Id: Idc74f1bb4653a89e134e87e46c21643977a33c82

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

diff --git a/src/main/java/com/android/tools/r8/ir/code/Assume.java b/src/main/java/com/android/tools/r8/ir/code/Assume.java
index 54cc778..53af635 100644
--- a/src/main/java/com/android/tools/r8/ir/code/Assume.java
+++ b/src/main/java/com/android/tools/r8/ir/code/Assume.java
@@ -295,7 +295,7 @@
             + ", type: "
             + src().getType()
             + ")";
-    assert src().getDefinition() == null || !src().getDefinition().isStringConcat()
+    assert src().isPhi() || !src().getDefinition().isStringConcat()
         : "Unexpected Assume value "
             + outValue()
             + " for StringConcat value "