Support codegen of empty do-while-loop

#KT-3009 In Progress

compiler/testData/codegen/box/controlStructures/emptyDoWhile.kt

@@ -0,0 +1,9 @@
+fun box(): String {
+    do while (false);
+
+    var x = 0
+    do while (x++<5);
+    if (x != 6) return "Fail: $x"
+
+    return "OK"
+}