Switch when to phi. (#172)

backend.native/tests/codegen/branching/when6.kt

@@ -0,0 +1,6 @@
+fun foo() {
+}
+
+fun main(args: Array<String>) {
+    if (true) foo() else foo()
+}

backend.native/tests/codegen/branching/when7.kt

@@ -0,0 +1,4 @@
+fun main(args: Array<String>) {
+    val b = args.size < 1
+    val x = if (b) Any() else throw Error()
+}