FIR checker: warn unnecessary non-null assertions

compiler/testData/diagnostics/tests/smartCasts/inference/kt1355.fir.kt

@@ -25,7 +25,7 @@ public data class Tag(public var tagName: String) {
                 attributes.remove("id")
             }
             else {
-                attributes["id"] = value!!
+                attributes["id"] = value<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
                 attributes["id"] = value
             }
         }

compiler/testData/diagnostics/tests/smartCasts/inference/kt2851.fir.kt

@@ -5,7 +5,7 @@ fun main() {
     val value: String? = ""
     if (value != null) {
         foo(Pair("val", value))
-        foo(Pair("val", value!!))
+        foo(Pair("val", value<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>))
         foo(Pair<String, String>("val", value))
     }
 }