Support codegen of empty while-loop

#KT-3009 In Progress

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

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