Fix coroutine-related VerifyError

The problem was that we spilled the `origin` variable (see test) as Object, because
we determined the type of merge(null, String) incorrectly.

 #KT-15973 Fixed

compiler/testData/codegen/box/coroutines/mergeNullAndString.kt

@@ -0,0 +1,38 @@
+// WITH_RUNTIME
+// WITH_COROUTINES
+import kotlin.coroutines.experimental.*
+import kotlin.coroutines.experimental.intrinsics.*
+
+fun builder(c: suspend () -> Unit) {
+    c.startCoroutine(EmptyContinuation)
+}
+
+var res = ""
+
+suspend fun foo(y: A?): String {
+    val origin: String? = y?.z
+    if (origin != null) {
+        baz(origin)
+        baz(origin)
+    }
+
+    return res
+}
+
+suspend fun baz(y: String): Unit = suspendCoroutineOrReturn { x ->
+    res += y[res.length]
+    x.resume(Unit)
+    COROUTINE_SUSPENDED
+}
+
+class A(val z: String)
+
+fun box(): String {
+    var result = ""
+
+    builder {
+        result = foo(A("OK"))
+    }
+
+    return result
+}