[ast-optimizer-branch] Reaching definitions and code generation [patch]

[Diego Novillo]

This patch adds a couple more SSA-based analyses and initial support
for inserting and replacing code:

- Reaching definitions and reached uses.

- Flow-sensitive insertion and replacement of tree nodes.

- Warnings for use of uninitialized variables.

The code has been tested with a transformation we're developing
internally.  It successfully bootstraps with the transformation
enabled.

Bootstrapped and regression tested on x86.  Committed to the AST
branch.

Diego.