FIR checker: warn useless as and is

compiler/testData/diagnostics/tests/smartCasts/castchecks/basicOn.fir.kt

@@ -14,7 +14,7 @@ fun g(a: SomeClass?) {
         a.hashCode()
         a.foo
         (a as? SomeSubClass)<!UNSAFE_CALL!>.<!>foo
-        (a as SomeSubClass).foo
+        (a <!USELESS_CAST!>as SomeSubClass<!>).foo
     }
     val b = (a as? SomeSubClass)?.foo
     if (b != null) {
@@ -22,7 +22,7 @@ fun g(a: SomeClass?) {
         a.hashCode()
         a.foo
         (a as? SomeSubClass)<!UNSAFE_CALL!>.<!>foo
-        (a as SomeSubClass).foo
+        (a <!USELESS_CAST!>as SomeSubClass<!>).foo
     }
     val c = a as? SomeSubClass
     if (c != null) {
@@ -33,4 +33,4 @@ fun g(a: SomeClass?) {
         c.hashCode()
         c.foo
     }
-}
+}