[m-rev.] diff: alternative fix for bug 264 test case

[Zoltan Somogyi]

On Sun, 22 Nov 2015 17:11:58 +1100, Mark Brown <mark at mercurylang.org> wrote:
> I've now checked these. Abstract unify is not a problem, but merge had
> a similar bug to the ones in inst matching; see the attached diff,
> which has been committed.

Have you tested the performance effect of this change, especially
on unusually large predicates? I suspect that the additional traversals
over insts could cause significant slowdowns. A possible way to avoid that
would be to add a field to inst_results to record whether the inst
contains what the new tests look for.

Zoltan.