constant folding: delete the folded constants

[hugomg]

Currently, the constant folding pass replaces the use of a constant, but leaves the original definition of the constant. For example, consider the following program:

local m = {}

local PI = 3.14

function m.area(r: float): float
    return PI*r*r
end

return m

If we look at the generated IR, we can see that the main function still has an useless x1<-3.14 definition. This is dead code and might be confusing to users because it looks like the the constant folding didn't happen, even though it did.

function main() {
    x1 <- 3.14
    x2 <- NewClosure(f2)
    x2.upvalues <-
    x3 <- NewTable(1)
    CheckGC()
    x3["area"] <- x2
    return x3
}
function f2(x1) {
    x3 <- 3.14 * x1
    x2 <- x3 * x1
    return x2
}