Fix a braino in the stack layout.

Also do some refactoring to have this code be just in CodeGenerator.

Change-Id: I88de109889138af8d60027973c12a64bee813cb7
diff --git a/test/401-optimizing-compiler/expected.txt b/test/401-optimizing-compiler/expected.txt
index 97492a4..d6ef64b 100644
--- a/test/401-optimizing-compiler/expected.txt
+++ b/test/401-optimizing-compiler/expected.txt
@@ -11,3 +11,4 @@
 Forced GC
 Forced GC
 Forced GC
+Forced GC