Cross-block deduplication in constant folding has correctness bugs.

[TomAFrench]

One of the components which has been causing issues on Noir syncs has been the changes made in #6499 so it's had to be pulled out of the syncs.
I opened this PR to test adding it in isolation and there's a number of test failures.

Sadly all of these tests rely on foreign calls so it's going to be a bit messy to get a nice reproduction case but we should find it and bring it back into this repo to fix the underlying issue.