Optimize gas charges

[vaivaswatha]

With #39 , we'll have gas_charge AST nodes, explicitly charging gas. Each such charge leads to a computation (comparison, conditional branch and a subtraction) at the LLVM-IR level (#54). However, these charges can potentially be combined into a single computation. For example, expressions leading to a straight line sequence (until a branch) with all static charges can be combined into one static charge.

Such opportunities to combine gas charges must be explored and optimized.

[vaivaswatha]

Another pattern to look for: When computing SizeOf a literal, if the literal's type guarantees a size statically, then replace that. This can help DCE do better. (such as eliminate dead loads since the only reason not-to is not knowing how much to charge).

[vaivaswatha]

With #87, more thought is needed on performing this optimization. For example:

gas_charge 10
let a = builtin div x y in
gas_charge 20
...

Cannot be replaced by a leading gas_charge 30 because a possible exception at the div will then impact the "gas remaining" (i.e., it'll be different than if the interpreter executed the code).