FIR: pass the explicit expected type to block type

This helps avoid adding redundant return Unit into block.

compiler/testData/ir/irText/stubs/builtinMap.fir.kt.txt

@@ -2,7 +2,7 @@ fun <K1 : Any?, V1 : Any?> Map<out K1, V1>.plus(pair: Pair<K1, V1>): Map<K1, V1>
   return when {
     <this>.isEmpty() -> mapOf<K1, V1>(pair = pair)
     else -> LinkedHashMap<K1?, V1?>(p0 = <this>).apply<LinkedHashMap<K1?, V1?>>(block = local fun LinkedHashMap<K1?, V1?>.<anonymous>() {
-      return <this>.put(p0 = pair.<get-first>(), p1 = pair.<get-second>()) /*~> Unit */
+      <this>.put(p0 = pair.<get-first>(), p1 = pair.<get-second>()) /*~> Unit */
     }
 )
   }