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

compiler/testData/diagnostics/tests/smartCasts/kt7561.fir.kt

@@ -0,0 +1,15 @@
+// Test for a potential byte code mistake for a postfix operation on a smart casted variable
+public fun box() : Int {
+    var i : Int? 
+    i = 10
+    val ii: Int = i
+    // k also should be Int
+    val k : Int = i++
+    // KT-7561: both i and i++ should be Int, otherwise VerifyError can arise here
+    // VerifyError reason: byte code tries to store (i++) result which is Int (smart cast)
+    // into a j which is Int?
+    val j = i++
+    // and m also
+    val m = ++i
+    return j + k + m + i + ii
+}