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"
+}

compiler/testData/diagnostics/tests/controlStructures/emptyIf.kt

@@ -0,0 +1,16 @@
+fun foo(x: Unit) = x
+
+fun test() {
+    if (false);
+    if (true);
+
+    val x = <!IMPLICIT_CAST_TO_UNIT_OR_ANY!>if (false)<!>;
+    foo(x)
+
+    val y: Unit = if (false);
+    foo(y)
+
+    foo({if (1==1);}())
+
+    return if (true);
+}

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
+}