Help with a DMD patch

[Vladimir Panteleev]

I'm trying to cut my teeth on DMD hacking with a simple 
optimization ( http://d.puremagic.com/issues/show_bug.cgi?id=9477 
), but not having much luck so far - the backend is rejecting my 
attempts with asserts here or there. Since the asserts have no 
error messages, and the code is very terse, often 
un-straightforward and lacking in comments, I have no idea if I'm 
feeding it bad input or if it's some latent bug.

Here's my work so far:
https://github.com/CyberShadow/dmd/compare/fix9477-work

Could a DMD guru have a look and tell me what I'm doing wrong?

Tests:
https://gist.github.com/CyberShadow/5177509