Undo 32-bit idempotent return type hack.

[vext01]

Requires ykjit/yk#1651

[vext01]

@ltratt do you think this one should go in too? Is it now trusted now that we think the problem was the ykllvm bug?

[ltratt]

"Yes but":

1. This will almost certainly have a merge conflict with Use a version number for idempotency. #107.
2. I'd ideally like to benchmark before/after this, just to see if it makes any difference. [I'm not expecting much of a difference, but am slightly curious.]

[vext01]

OK. I'll hold off.

I don't expect a large performance change, but let's see. If anything the removal of bitcast instructions should be in our favour.

[ltratt]

Please merge in master and address the merge conflict. Because this one's quite subtle, I think a force push is not the way to go here.

[vext01]

I think this is correct. The lua tests are passing.

[ltratt]

Looks good! Please squash.

[vext01]

squashed