[FIR] Create test case to validate KT-56988 is working as expected

^KT-56988 Fixed

compiler/testData/diagnostics/testsWithStdLib/smartcasts/whileLoopNotNull.fir.kt

@@ -0,0 +1,22 @@
+// FIR_DUMP
+
+fun Int?.swap(): Int = 1
+fun Int.swap(): Int? = null
+
+var result = false
+
+fun b(): Boolean {
+    result = !result
+    return result
+}
+
+fun test() {
+    var x: Int? = 1
+    if (x != null) {
+        while (b()) {
+            val tmp = x.swap()
+            x = tmp
+        }
+        x.plus(1)
+    }
+}