JVM IR: Avoid direct invokes in callable reference tests

Due to the direct invoke optimization, most callable reference tests
were not generating callable references/lambdas.

compiler/testData/codegen/box/multiDecl/forIterator/MultiDeclForValCaptured.kt

@@ -7,7 +7,7 @@ class C(val i: Int) {
 fun doTest(l : ArrayList<C>): String {
     var s = ""
     for ((a, b) in l) {
-      s += {"$a:$b;"}()
+      s += {"$a:$b;"}.let { it() }
     }
     return s
 }

compiler/testData/codegen/box/multiDecl/forIterator/longIterator/MultiDeclForComponentExtensionsValCaptured.kt

@@ -5,7 +5,7 @@ operator fun Long.component2() = this + 2
 fun doTest(l : ArrayList<Long>): String {
     var s = ""
     for ((a, b) in l) {
-      s += {"$a:$b;"}()
+      s += {"$a:$b;"}.let { it() }
     }
     return s
 }