[llvm] [SLP][NFC] Extract VectorizableTree into own ForestTree class (PR #222805)

[Ryan Buchner via llvm-commits]

bababuck wrote:

Will try to keep this rebased and up to date, though I expect every update to the `SLP` will conflict with this MR so will quickly go stale.

https://github.com/llvm/llvm-project/pull/222805