[llvm] [SLP]Represent SLP graph as a tree (PR #126771)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> @alexey-bataev It seems odd to insist on handling vectorizable DAGs as individual sub-trees with external users. This seems to take us further from being able to eventually cost multiple roots in one go (i.e. trees with share sub-expressions). Practical engineering is filed with tradeoffs, so if you're reasonable sure this is the right direction, please continue. I'm just noting the question.

We still support this, but not as a DAG, but as a smart matching of the nodes, so we don't miss anything here. It could be a problem before, but not now, since we support effective matching for gathered nodes against vector nodes.
Shared sub-expression vectorization is also on my list and I'm working on, os we don't miss anything here too.

https://github.com/llvm/llvm-project/pull/126771