[llvm] [DomTree] Reduce number of hash table lookups (NFC) (PR #73097)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

Awesome, thanks for looking into this @nikic. I remember a large percentage of the total domtree computation/update time has always been spent inside dense maps.

Just to confirm, here we are effectively trading more memory usage (one new vector) for better runtime performance (fewer hash map lookups)? Is the memory usage difference insignificant enough, or could we have someone with the opposite constraints come and suggest a reverse change?

https://github.com/llvm/llvm-project/pull/73097