Load pointer and length of a list before creating the block for the deallocation loop.

[primoly]

Currently the block for the deallocation loop gets generated before the pointer and the length of the list.

In the haskell generator this leads to the problem where the variables that store the pointer/length are pushed to the inner block rather than the outer one (since I always just push all code to the last block on the “block-stack”) making them inaccessible for the loop setup itself. Both Rust and C do not store the pointer/length in variables, just pushing the code to load them onto the results directly.

This solves that issue by simply getting them before the inner block is created. I don’t think this will break any other generators, tho I can’t test most of them on my machine.

Alternatively I could keep track of the current active block manually rather than assuming it’s the last one but I would prefer this solution instead if possible.

[alexcrichton]

Seems reasonable to me, thanks! The CI failures should be fixed by #998

[alexcrichton]

If you're up for rebasing this it should pass CI now and I can queue for merge