[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/smartCasts/fieldPlus.fir.kt

@@ -0,0 +1,14 @@
+// !DIAGNOSTICS: -UNNECESSARY_NOT_NULL_ASSERTION
+// Advancement of KT-9126
+
+class My(val x: Int?) {
+    operator fun plus(y: My) = if (this.x != null) this else y
+}
+
+fun foo() {
+    var y: My? = My(42)
+    if (y!!.x != null) {
+        y = My(null)
+        (y + My(0)).x.hashCode()
+    }
+}