Support warning about useless cast on safe cast

#KT-13348 Fixed

compiler/testData/diagnostics/tests/cast/UselessSafeCast.kt

@@ -0,0 +1,22 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER, -UNUSED_VARIABLE
+
+fun test(x: Int?) {
+    val a1 = x <!USELESS_CAST!>as? Int<!>
+    val a2 = x <!USELESS_CAST!>as? Int?<!>
+    val a3 = x as? Number
+    val a4 = x as? Number?
+    val a5: Int? = x <!USELESS_CAST!>as? Int<!>
+    val a6: Number? = x <!USELESS_CAST!>as? Int<!>
+    val a7: Number? = 1 <!USELESS_CAST!>as? Number<!>
+
+    run { x <!USELESS_CAST!>as? Int<!> }
+    run { x as? Number }
+
+    foo(x as? Number)
+
+    if (x is Int) {
+        val b = x as? Int
+    }
+}
+
+fun foo(x: Number?) {}

compiler/testData/diagnostics/tests/cast/UselessSafeCast.txt

@@ -0,0 +1,4 @@
+package
+
+public fun foo(/*0*/ x: kotlin.Number?): kotlin.Unit
+public fun test(/*0*/ x: kotlin.Int?): kotlin.Unit