[OldPM] Pass manager: run SROA after (simple) loop unrolling
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 allocas remained, causing about +160% perf regression.
suggests that this has geomean compile-time cost of +0.08%.
Reviewed By: nikic, xbolva00
Differential Revision: https://reviews.llvm.org/D87972