backend: Use IrComposite instead of IrBlock in headers of 'for' loops

backend.native/tests/build.gradle

@@ -348,6 +348,15 @@ task codegen_controlflow_for_loops_nested(type: RunKonanTest) {
             "00 10 20 \n"
 }
 
+task codegen_controlflow_for_loops_coroutines(type: RunKonanTest) {
+    source = "codegen/controlflow/for_loops_coroutines.kt"
+    goldValue = "before: 0 after: 0\n" +
+            "before: 2 after: 2\n" +
+            "before: 4 after: 4\n" +
+            "before: 6 after: 6\n" +
+            "Got: 0 2 4 6\n"
+}
+
 task local_variable(type: RunKonanTest) {
     source = "codegen/basics/local_variable.kt"
 }

backend.native/tests/codegen/controlflow/for_loops_coroutines.kt

@@ -0,0 +1,12 @@
+import kotlin.coroutines.experimental.*
+
+fun main(args: Array<String>) {
+    val sq = buildSequence {
+        for (i in 0..6 step 2) {
+            print("before: $i ")
+            yield(i)
+            println("after: $i")
+        }
+    }
+    println("Got: ${sq.joinToString(separator = " ")}")
+}