FIR checker: warn unnecessary safe calls

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

@@ -17,5 +17,5 @@ fun test1(a: A) {
 fun test2() {
     val a: A? = null;
     assert((a as B).bool())
-    a?.bool()
+    a<!UNNECESSARY_SAFE_CALL!>?.<!>bool()
 }

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

@@ -5,7 +5,7 @@
 
 fun test1(s: String?) {
     assert(s!!.isEmpty())
-    s?.length
+    s<!UNNECESSARY_SAFE_CALL!>?.<!>length
 }
 
 fun test2(s: String?) {
@@ -21,7 +21,7 @@ fun test3(s: String?) {
 fun test4() {
     val s: String? = null;
     assert(s!!.isEmpty())
-    s?.length
+    s<!UNNECESSARY_SAFE_CALL!>?.<!>length
 }
 
 fun test5() {