Cranelift: consider adding a loop-peeling optimization pass

That is, a pass that unrolls one iteration of the loop body just before the actual loop. This is often beneficial because loop bodies might contain conditional code that only executes on the first iteration.

There could be additional benefit for Cranelift in particular: right now, Cranelift will deduplicate various kinds side-effecting-but-idempotent instructions (such as `trapz`s) but will not move those instructions outside of their original blocks and hoist them just above the loop even when it would be valid to do so. But if we have already unrolled the first iteration of a loop, then that unrolled first iteration will dominate the rest of the loop and when we dedupe those instructions from inside the loop body to their unrolled counterparts, we are effectively doing exactly that code motion we wanted to do before but could not.

Random tidbits:

* Could we do this at the same time as the aegraphs pass? I *think* so? But it would be in the same traversal, not actually in ISLE, similar to eg the alias analysis optimization.

* We will need some heuristics for when to peel loops or not. Don't think we want the code bloat of peeling every single loop in the whole program. Maybe just innermost loops? Maybe just loops of up to a certain size? Lots of experimenting to be done in this regard.