[llvm] [SLP]Vectorize unique scalars of splat gather nodes as separate subtrees (PR #218250)

[Ryan Buchner via llvm-commits]

bababuck wrote:

> I think it is unrelated issue, will fix it in another PR

My understanding is that it is caused by the newly added `SplatGatheredScalarsRoots` not being cleared when it gets trimmed.

https://github.com/llvm/llvm-project/pull/218250