ConstantExpr lowering pass for code generators

Following up on the '[LLVMdev] Getelementptr woes' thread:
http://mail.cs.uiuc.edu/pipermail/llvmdev/2004-June/001252.html

I'm happy to say that Vladimir Prus contributed a constant expression
lowering pass for instruction selectors to use. This pass makes
instruction selection both easier and less error prone.

Thanks Vladimir, and sorry for being such a butthead about it earlier on
the list. :slight_smile:

-Chris