backend: fix minor bug in evaluateWhen

Also add the corresponding test 'when9'.

backend.native/tests/build.gradle

@@ -330,6 +330,11 @@ task when8(type: RunKonanTest) {
     goldValue = "true\n"
 }
 
+task when9(type: RunKonanTest) {
+    goldValue = "Ok\n"
+    source = "codegen/branching/when9.kt"
+}
+
 task when_through(type: RunKonanTest) {
     source = "codegen/branching/when_through.kt"
 }

backend.native/tests/codegen/branching/when9.kt

@@ -0,0 +1,10 @@
+fun main(args: Array<String>) {
+    foo(0)
+    println("Ok")
+}
+
+fun foo(x: Int) {
+    when (x) {
+        0 -> 0
+    }
+}