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/diagnostics/tests/dataFlow/EmptyIf.kt

@@ -0,0 +1,22 @@
+fun f1(s: String?) {
+    if (s!! == "");
+    s : String
+}
+
+fun f2(s: Number?) {
+    if (s is Int);
+    <!TYPE_MISMATCH!>s<!> : Int
+    if (s as Int == 42);
+    s : Int
+}
+
+fun f3(s: Number?) {
+    if (s is Int && s as Int == 42);
+    <!TYPE_MISMATCH!>s<!> : Int
+}
+
+fun f4(s: Int?) {
+    var u = <!IMPLICIT_CAST_TO_UNIT_OR_ANY!>if (s!! == 42)<!>;
+    if (u == Unit.VALUE) u = if (s == 239);
+    return u
+}