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

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 = " ")}")
+}