[llvm] [Support] Use bump ptr allocator for domtree nodes (PR #102516)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Yeah, looks like this was accidentally introduced when this was spit into a separate analysis pass in [2d5487c](https://github.com/llvm/llvm-project/commit/2d5487cf446f9a58f6d2f76df33102423c83f285).

Cc @teresajohnson, is this something worth following up on?

https://github.com/llvm/llvm-project/pull/102516