FIR. Fix smart-cast expression unwrapping in plus with assign resolve

compiler/fir/analysis-tests/testData/resolve/expresssions/plusWithAssignSmartCast.kt

@@ -0,0 +1,11 @@
+class Foo {
+    var bar: String? = null
+
+    fun addToBar(other: String) {
+        if (bar == null) {
+            bar = other
+        } else {
+            bar += " $other"
+        }
+    }
+}