Dead code elimination

#KT-6602 Fixed
 #KT-6305 Fixed
 #KT-5656 Fixed

compiler/testData/codegen/boxWithStdlib/deadCodeElimination/emptyVariableRange.kt

@@ -0,0 +1,12 @@
+import kotlin.test.assertEquals
+
+fun foo(): Int {
+    return 1
+    // val xyz has empty live range because everything after return will be removed as dead
+    val xyz = 1
+}
+
+fun box(): String {
+    assertEquals(1, foo())
+    return "OK"
+}