[FIR] DFA: add missing unwrap for FirDesugaredAssignmentValueReferenceExpression

^KT-60319 Fixed
^KT-61407 Fixed

compiler/testData/diagnostics/tests/dataFlow/desugaredAssignment.fir.kt

@@ -0,0 +1,17 @@
+// WITH_STDLIB
+package usage
+
+class MyClass1
+open class ABC {
+    open val nestedBlocks = ArrayList<MyClass1>()
+
+    fun makeInjectionBlocks() {
+        val l: List<Any> = listOf(1)
+        for (b in l) {
+            when (b) {
+                is Int -> <!ASSIGNMENT_TYPE_MISMATCH!><!VAL_REASSIGNMENT!>nestedBlocks<!> += b<!>
+                else -> {}
+            }
+        }
+    }
+}