FIR checker: warn unnecessary non-null assertions

compiler/testData/diagnostics/testsWithStdLib/assert/safeCall.fir.kt

@@ -10,7 +10,7 @@ fun test1(s: String?) {
 
 fun test2(s: String?) {
     assert(s!!.isEmpty())
-    s!!.length
+    s<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.length
 }
 
 fun test3(s: String?) {
@@ -27,7 +27,7 @@ fun test4() {
 fun test5() {
     val s: String? = null;
     assert(s!!.isEmpty())
-    s!!.length
+    s<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.length
 }
 
 fun test6() {

compiler/testData/diagnostics/testsWithStdLib/regression/kt10001.fir.kt

@@ -1,5 +1,5 @@
 fun foo1(p: Pair<Int?, Int>): Int {
-    if (p.first != null) return p.first!!
+    if (p.first != null) return p.first<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
     return p.second
 }