FIR checker: warn unnecessary non-null assertions

compiler/tests-spec/testData/diagnostics/notLinked/contracts/analysis/smartcasts/pos/14.fir.kt

@@ -55,7 +55,7 @@ fun case_1(value_1: Int?) {
 
 // TESTCASE NUMBER: 2
 fun case_2(value_1: Int?) {
-    if (!contracts.case_2(value_1)!!) {
+    if (!contracts.case_2(value_1)<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>) {
         value_1<!UNSAFE_CALL!>.<!>inv()
     }
 }

compiler/tests-spec/testData/diagnostics/notLinked/contracts/analysis/smartcasts/pos/2.fir.kt

@@ -159,6 +159,6 @@ class case_10_class {
  */
 fun case_11(value_1: Any?, value_2: Any?, value_3: Any?) {
     funWithReturnsAndInvertCondition(value_1 !is String || value_2 !is Number || value_3 !is Any?)
-    println(value_1!!.length)
+    println(value_1<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.length)
     println(value_2?.toByte())
 }