Removing the ReorderStatements pass

This is a copy-pase of the commit message from 61badb32562e0e7fd64464cc0dffadcebf6cc76b.

The purpose of the ReorderStatements pass was to improve locality of code the futhark IR. The motivating problem was some double buffers in OptionPricing which would only be copied to at the very end of the program. This lead to some arrays being in-use for much longer than needed, preventing the reuse of their memory blocks. More details here: https://munksgaard.me/technical-diary/2020-10-07.html#orgbbfe6a7

Unfortunately, I have not seen any actual benefits from introducing this pass. In the OptionPricing benchmark specifically, it turned out that moving final double-buffer copy up did not yield any additional opportunities for memory block merging. And in fact, some benchmarks actually use more memory after having been reordered, because of the simplistic way this pass is implemented. As an example, consider the following code:

1: let map_res@ext_mem = ...
2: let res_mem = alloc ...
3: let res@res_mem = copy(map_res)
4: in (res_mem, res)

The ReorderStatements algorithm would start by looking at the result of the body, (res_mem, res) and determine that both would need to be computed. Because it does not know the difference between a memory block and an array it arbitrarily decides to compute res_mem first, resulting in the following reordered code:

1: let res_mem = alloc ...
2: let map_res@ext_mem = ...
3: let res@res_mem = copy(map_res)
4: in (res_mem, res)

But now there's no opportunity to merge resmem with extmem, because their lifetimes overlap.

I'm afraid overcoming this problem would require a significant rewrite of ReorderStatements, and I'm not even sure we'd get any performance benefits out of it.

Therefore, I'm removing the pass for now. We can always dig it up again later.