prebirth: Wrap `if' applications

* build-aux/bootstrap/prebirth.js (fnmap)[if]: Wrap generated code
    in self-executing function.

build-aux/bootstrap/prebirth.js

@@ -543,10 +543,13 @@ class Compiler
 const fnmap = {
     'js:console': 'console.log',
 
-    // simple if statement with optional else
+    // simple if statement with optional else, wrapped in a self-executing
+    // function to simplify code generation (e.g. returning an if)
     'if': ( [ pred, t, f ], stoes ) =>
-        `if (${stoes(pred)}) {${stoes(t)};} ` +
-        ( ( f === undefined ) ? '' : `else {${stoes(f)};}` ),
+        "(function(){" +
+          `if (${stoes(pred)}){return ${stoes(t)};}` +
+          ( ( f === undefined ) ? '' : `else{return ${stoes(f)};}` ) +
+        "})()",
 
     // (let ((binding val) ...) ...body), compiled as a self-executing
     // function which allows us to easily represent the return value of the