constant folding: fold non-trivial constants

[hugomg]

It would be nice if our constant folding pass could propagate things that aren't constant literals. For example:

local PI = 3.14
local TAU = 2*PI

function m.f()
   -- something using TAU
end

[srijan-paul]

This kind of constant folding should come after constant propagation, no?

[hugomg]

I think it might want to interleave them. First we constant fold the outermost function. Then, we look at all the initUpvales inside it and see which ones are constant and propagate those constants into the inner functions. Then we repeat the process for the inner functions.

[srijan-paul]

Makes sense. Perhaps we do N passes until no more constants can be propagated / folded.