[FIR] Add test cases to prove resolution of backlog tickets

^KT-41728 Fixed
^KT-46586 Fixed
^KT-49696 Fixed
^KT-51160 Fixed
^KT-53308 Fixed

compiler/testData/diagnostics/tests/controlFlowAnalysis/deadCode/smartCastAfterDeadCode.kt

@@ -0,0 +1,10 @@
+// WITH_STDLIB
+// ISSUE: KT-41728
+
+fun foo(a: String?) {
+    if (a == null) {
+        return
+        <!UNREACHABLE_CODE!>"hi".length<!>
+    }
+    a<!UNSAFE_CALL!>.<!>length
+}