[llvm] [CodeGen] Speed up ReachingDefAnalysis (NFC) (PR #100913)

[Nikita Popov via llvm-commits]

nikic wrote:

Right, RDA is more constrained by memory usage than performance. This patch increases max-rss by 2-4% (see "link only" results on https://llvm-compile-time-tracker.com/compare.php?from=4202634886b9b8f1ee74713a8ed8c86941891853&to=afa915730473facf8e9879e00ee0fa5d67e13731&stat=max-rss&linkStats=on).

I have some old patches that I never got around to landing to swap the order of reg unit and BB number storage (basically https://github.com/nikic/llvm-project/commits/perf/reaching-defs-6/) which showed promising results at the time.

https://github.com/llvm/llvm-project/pull/100913