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/unit/sillyThings.kt

@@ -30,11 +30,11 @@ fun box(): String {
     val tmp = Unit
     {
 
-    }()
+    }.let { it() }
 
     val tmp2 = {
 
-    }()
+    }.let { it() }
 
     val tmp3 = Blumbs()
     tmp3.t = Unit