[Tree Optimizer] Unnecessary Let deletion

[oflatt]

An important optimization to enable other optimizations is to get rid of unnecessary lets. In the case of pure computations, let bindings are not necessary and they should be removed- maybe destructively?

For impure computations that are used just once, the let binding is not necessary. However, we must be careful to avoid bugs since later the e-class could have representatives that use it twice.

[yihozhang]

Let bindings save duplicate evaluations even for pure computations, e.g.

let x = sum(1, 10000) in
x+x

vs

sum(1, 10000) + sum(1, 10000) 

so we need to be careful here.

[oflatt]

Yeah, that's a great point. What if we used DAG-based extraction, even for trees? It could count things multiple times when they are impure

[yihozhang]

Related to gamma/switch pull in