[branch]: Structure aliasing branch created

[Daniel Berlin <dberlin at dberlin dot org>]

The structure-aliasing-branch has been created (or rather, is in the 
middle of being tagged).

I'll be using this branch to do work on making structure aliasing work 
better (including adding support for v_may_defs of parts of variables, 
etc).

Anything that bootstraps can be committed, but please let me know if 
you plan on working on something, so we don't duplicate work.

The plan looks something like this right now:

1. Modify V_MAY_DEFS (and possibly vuses) to have a size and offset 
argument
2. Modify the renamer to DTRT in regard to renaming versions for uses 
of different offsets (IE track reaching definitions for various offsets 
of structures).
3. Modify the operand getting code to create the structure killing 
vuses for only the actually killed portions of the structure.
4. Add the new alias analyzer to the branch that can determine what 
parts of structures and arrays  are aliased, and what p->a and p->b and 
a[0] and a[1] actually point to.
4. Use the new alias analyzer to create partial v_may_defs/vuses

I'll add this to the development branches list in a few minutes