[FIR] Prohibit is-checks and casts for unrelated nullable types

This decision was made after a discussion
in Slack.

compiler/testData/diagnostics/tests/cast/NullableToNullable.fir.kt

@@ -0,0 +1,7 @@
+// From KT-13324: always succeeds
+val x = null as String?
+// From KT-260: sometimes succeeds
+fun foo(a: String?): Int? {
+    val c = a <!USELESS_CAST!>as? Int?<!>
+    return c
+}

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

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // From KT-13324: always succeeds
 val x = null as String?
 // From KT-260: sometimes succeeds