FIR checker: warn unnecessary safe calls

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

@@ -5,7 +5,7 @@ fun test11() {
 
 fun test12() {
     fun Any.bar(i: Int) {}
-    todo()?.bar(1)
+    todo()<!UNNECESSARY_SAFE_CALL!>?.<!>bar(1)
 }
 
-fun todo(): Nothing = throw Exception()
+fun todo(): Nothing = throw Exception()

compiler/testData/diagnostics/tests/controlFlowAnalysis/throwInLambda.fir.kt

@@ -7,6 +7,6 @@ fun foo(): String {
 }
 fun bar(): String {
     val x = fn() ?: return ""
-    val y = x?.let { throw Exception() } ?: "unreachable"
+    val y = x<!UNNECESSARY_SAFE_CALL!>?.<!>let { throw Exception() } ?: "unreachable"
     return y
-}
+}