Support empty if-statements

Type-check "if (...) ;" to Unit, report "implicit cast to Unit", propagate data
flow info out of its condition

 #KT-2478 Fixed

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

@@ -0,0 +1,12 @@
+fun box(): String {
+    if (true);
+    if (false);
+    val iftrue = if (true);
+    val iffalse = if (false);
+
+    var state = 0
+    val k = if (state++==1);
+    if (state != 1) return "Fail: $state"
+
+    return "OK"
+}