[llvm] [SLP] Do not skip tiny trees with gathered loads to vectorize (PR #190040)

[Alex Bradbury via llvm-commits]

asb wrote:

I can also confirm my shared reproducer is fixed in current HEAD. Thanks for the rapid fix.

Small request for the future: could fixed up relands like #190181 mention the original PR please? It's helpful to keep them linked, both as it's more discoverable in GitHub and it might be helpful in the future to see how the commit changed vs the original PR.

https://github.com/llvm/llvm-project/pull/190040