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

[Nikita Popov via llvm-commits]

nikic wrote:

> 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?

The additional memory usage here is basically one pointer per node and only used temporarily, so I don't think it will have any measurable impact on peak memory usage. (Far below the noise threshold for my max-rss measurements.)

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