FIR checker: warn unnecessary non-null assertions

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/AssertNotNull.fir.kt

@@ -5,35 +5,35 @@ fun main() {
     val a : Int? = null
     val b : Int? = null
     checkSubtype<Int>(a!!)
-    a!! + 2
-    a!!.plus(2)
-    a!!.plus(b!!)
-    2.plus(b!!)
-    2 + b!!
+    a<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!> + 2
+    a<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.plus(2)
+    a<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.plus(b!!)
+    2.plus(b<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>)
+    2 + b<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
 
     val c = 1
-    c!!
+    c<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
 
     val d : Any? = null
 
     if (d != null) {
-        d!!
+        d<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
     }
 
     // smart cast isn't needed, but is reported due to KT-4294
     if (d is String) {
-        d!!
+        d<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
     }
 
     if (d is String?) {
         if (d != null) {
-            d!!
+            d<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
         }
         if (d is String) {
-            d!!
+            d<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
         }
     }
 
-    val f : String = a!!
-    <!INAPPLICABLE_CANDIDATE!>checkSubtype<!><String>(a!!)
+    val f : String = a<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
+    <!INAPPLICABLE_CANDIDATE!>checkSubtype<!><String>(a<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>)
 }