FIR checker: warn unnecessary non-null assertions

compiler/testData/diagnostics/tests/controlFlowAnalysis/deadCode/deadCodeInInnerExpressions.fir.kt

@@ -2,7 +2,7 @@
 
 fun testCompound() {
     operator fun Nothing.get(i: Int) {}
-    todo()!![12]
+    todo()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>[12]
 }
 
 fun testCompound1() {
@@ -10,4 +10,4 @@ fun testCompound1() {
     (todo() * "")[1]
 }
 
-fun todo(): Nothing = throw Exception()
+fun todo(): Nothing = throw Exception()