Fix false "useless cast" when target type is flexible

#KT-17820 Fixed

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

@@ -0,0 +1,26 @@
+// FILE: Foo.java
+
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+public class Foo {
+    static Foo create() { return null; }
+
+    @Nullable
+    static Foo createN() { return null; }
+
+    @NotNull
+    static Foo createNN() { return null; }
+}
+
+// FILE: sample.kt
+
+fun test() {
+    Foo.create() as Foo
+    Foo.createN() as Foo
+    Foo.createNN() <!USELESS_CAST!>as Foo<!>
+
+    Foo.create() <!USELESS_CAST!>as Foo?<!>
+    Foo.createN() <!USELESS_CAST!>as Foo?<!>
+    Foo.createNN() <!USELESS_CAST!>as Foo?<!>
+}

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

@@ -0,0 +1,15 @@
+package
+
+public fun test(): kotlin.Unit
+
+public open class Foo {
+    public constructor Foo()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+
+    // Static members
+    public/*package*/ open fun create(): Foo!
+    @org.jetbrains.annotations.Nullable public/*package*/ open fun createN(): Foo?
+    @org.jetbrains.annotations.NotNull public/*package*/ open fun createNN(): Foo
+}