[llvm] [SLP]Drop unprofitable splat gather subtrees (PR #221717)

[Tomas Matheson via llvm-commits]

tommat01 wrote:

> Thanks for repros, helped a lot, should be fixed

@alexey-bataev thanks again! Unfortunately there are still regressions in the original benchmarks.

[x264-complete.ll.txt](https://github.com/user-attachments/files/32249075/x264-complete.ll.txt) still doesn't vectorise as well as it did before https://github.com/llvm/llvm-project/pull/220250. 

spec/lbm is partially improved but also does not vectorise as well as pre-220250. I can't post the spec source so here is another attempt at a reproducer: 
[lbm-splat-residual.ll.txt](https://github.com/user-attachments/files/32515020/lbm-splat-residual.ll.txt)


https://github.com/llvm/llvm-project/pull/221717