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

[Tomas Matheson via llvm-commits]

tommat01 wrote:

Great, thanks. x264 is fixed but LBM not quite. I have another reproducer: 
[lbm-splat-remaining.ll.txt](https://github.com/user-attachments/files/32564131/lbm-splat-remaining.ll.txt)

There also seems to be an orthogonal issue with LBM related to https://github.com/llvm/llvm-project/pull/215840, I will try to isolate it.

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