private-kernel-lib tests are failing on Noir master

[TomAFrench]

See CI run which runs nargo test on the protocol circuit tests: https://github.com/noir-lang/noir/actions/runs/12091844455/job/33720813101?pr=6660

These are all tests which don't make use of custom foreign calls so they should be relatively simple to pull out into this repository, minimise and create a regression test.

[asterite]

Debugging this a bit, it seems the issue is with instruction hoisting in constant folding. There's code like this:

        if num_note_hashes != 0 {
            let x = num_note_hashes - 1;

and we get attempt to subtract with overflow likely because that subtraction is done even if num_note_hashes is zero. I still need to figure out exactly what's the issue, but disabling that hoisting fixes it.

[asterite]

@TomAFrench @jfecher We are hoisting any instruction that doesn't have side effects. But sub (and other binary math operations?) can lead to underflow/overflow so we can't always hoist it: if that sub can only be done after we check that a value is not zero, doing it before that will lead to that panic.

I guess we can consider Add, Sub, Mul and Div as having side effects? Alternatively we can drop the has_side_effects name and just use can_be_hoisted.

What do you think?

[TomAFrench]

Answered in #6672 but yeah we should disable this optimization on any instructions which break it so that we can get tests passing (as it's a relatively recent optimization this won't cause a deal-breaking regression). We can then look at how to optimize more of these instructions in followups.