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

[Philip Reames via llvm-commits]

preames 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.  

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