J2K: Remove unnecessary casts of non-nullable expression to nullable type after conversion

#KT-11543 Fixed

j2k/testData/fileOrElement/nullability/notNullCast.kt

@@ -0,0 +1,31 @@
+// ERROR: Type mismatch: inferred type is Passenger.PassChild? but Passenger.PassChild was expected
+// ERROR: Type mismatch: inferred type is Passenger.PassChild? but Passenger.PassChild was expected
+class Passenger {
+    open class PassParent
+
+    class PassChild : PassParent()
+
+    fun provideNullable(p: Int): PassParent? {
+        return if (p > 0) PassChild() else null
+    }
+
+    fun test1() {
+        val pass = provideNullable(1)!!
+        accept1(pass as PassChild)
+    }
+
+    fun test2() {
+        val pass = provideNullable(1)
+        if (1 == 2) {
+            assert(pass != null)
+            accept2(pass as PassChild?)
+        }
+        accept2(pass as PassChild?)
+    }
+
+    fun accept1(p: PassChild) {
+    }
+
+    fun accept2(p: PassChild) {
+    }
+}