[llvm] [SLP]Drop auxiliary subtrees to pass the VF=2 instruction-count veto (PR #224108)

[Ryan Buchner via llvm-commits]

https://github.com/bababuck approved this pull request.

LGTM. Though if I understand correctly, if we have 4 different splat values, but one of the user nodes gets trimmed, we don't recreate `SplatGatheredScalarsRoots` with VF=3 and dropping that one value? 

https://github.com/llvm/llvm-project/pull/224108