Mark only unreachable parts of element if it has reachable parts

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

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

@@ -0,0 +1,18 @@
+fun testAssignment() {
+    var <!UNUSED_VARIABLE!>a<!> = 1
+    <!UNREACHABLE_CODE!>a =<!> todo()
+}
+
+fun testVariableDeclaration() {
+    <!UNREACHABLE_CODE!>val <!UNUSED_VARIABLE!>a<!> =<!> todo()
+}
+
+fun testPlusAssign() {
+    fun Int.plusAssign(<!UNUSED_PARAMETER!>i<!>: Int) {}
+
+    var a = 1
+    a <!UNREACHABLE_CODE!>+=<!> todo()
+}
+
+
+fun todo() = throw Exception()