Mark only unreachable parts of element if it has reachable parts

like for 'return todo()' mark only 'return'

compiler/testData/diagnostics/tests/inference/regressions/kt2445.kt

@@ -1,3 +1,4 @@
+// !DIAGNOSTICS: -UNREACHABLE_CODE
 //KT-2445 Calling method with function with generic parameter causes compile-time exception
 package a
 
@@ -7,4 +8,4 @@ fun main(args: Array<String>) {
     }
 }
 
-fun test<R>(callback: (R) -> Unit):Unit = <!UNREACHABLE_CODE!>callback<!>(null!!)
+fun test<R>(callback: (R) -> Unit):Unit = callback(null!!)

compiler/testData/diagnostics/tests/inference/regressions/kt2838.kt

@@ -1,3 +1,4 @@
+// !DIAGNOSTICS: -UNREACHABLE_CODE
 //KT-2838 Type inference failed on passing null as a nullable argument
 package a
 
@@ -9,9 +10,9 @@ fun test(a: Int) {
     bar(a, <!NULL_FOR_NONNULL_TYPE!>null<!>)
 }
 fun test1(a: Int) {
-    <!UNREACHABLE_CODE!>foo<!>(a, throw Exception())
+    foo(a, throw Exception())
 }
 
 fun test2(a: Int) {
-    <!UNREACHABLE_CODE!>bar<!>(a, throw Exception())
+    bar(a, throw Exception())
 }