[sil-combine] Add peephole: alloc_ref/set_deallocating/dealloc_ref ->remove these instructions

[swiftix]

Discovered this missing peephole while looking at the performance of #7420

[swiftix]

@swift-ci please smoke test and merge

[palimondo]

If this should have any performance or correction impact, shouldn’t this be accompanied with a couple of tests?

(Not to nag, I’m just looking at the issue underlying SR-413 which was referenced in #7420 and I’m trying to understand the impact of this change. Has this missed 3.1 release? I can see it in master and swift-4.0-branch.)

[swiftix]

@palimondo The commit contains a SIL test, which checks that the peephole is performed. The impact on performance is very small, so there is no dedicated benchmark. And this is quite typical. Many peepholes in sil-combine are very small, so that any single one does not have a big performance impact in many cases. But together they often result in measurable performance improvements. Thus, sil-combine peepholes are usually tested at the SIL level.