I have stumbled into this pretty accidentally, when rewriting some spaghetti-like code
into something more structured, which involved using some `std::array<>`s.
And to my surprise, the `alloca`s remained, causing about `+160%` perf regression.
suggests that this has geomean compile-time cost of `+0.08%`.
This fixes [[ https://bugs.llvm.org/show_bug.cgi?id=40011 | PR40011 ]], [[ https://bugs.llvm.org/show_bug.cgi?id=42794 | PR42794 ]] and probably some other reports.