Local/Global value numbering

[rachitnigam]

There are three possible optimizations that can benefit code generated from Dahlia:

1. Constant folding

let x = 5; // X is never written to
x + x;
10 * x

2. Unused writes

x := 10; // x is never read from

3. Redundant stores

let x = 10;
y = x; // x is never written to after this point

This will take some thought to implement since we have to reverse-engineer a group's intent in the same way #177 did (to know which group constitute a useless write).