fix: val can be reassigned through backing field inside its own getter

compiler/testData/diagnostics/tests/controlFlowAnalysis/backingFieldInsideGetter.kt

@@ -0,0 +1,22 @@
+package a
+
+import java.util.HashSet
+
+val a: MutableSet<String>? = null
+    get() {
+        if (a == null) {
+            $a = HashSet()
+        }
+        return a
+    }
+
+class R {
+    val b: String? = null
+        get() {
+            if (b == null) {
+                $b = "b"
+            }
+            return b
+        }
+}
+