KT-3273 Return type coalesces to Tuple0 in when/if nested clauses

#KT-3273 Fixed

compiler/testData/codegen/regressions/kt3273.kt

@@ -0,0 +1,17 @@
+public fun testCoalesce() {
+   val value: String = when {
+       true -> {
+          if (true) {
+             "foo"
+          } else {
+             "bar"
+          }
+       }
+       else -> "Hello world"
+    }
+}
+
+fun box(): String {
+    testCoalesce()
+    return "OK"
+}