[llvm] [SLP]Reduce number of alternate instruction, where possible (PR #123360)

[Hans Wennborg via llvm-commits]

zmodem wrote:

@shiltian for those tests. It's not clear to me how they depend on the change I reverted exactly.

It also looks like Alexey re-landed in 10085390c6d4d0df04bf412f4c9ce7ebaaa5e3f3

https://github.com/llvm/llvm-project/pull/123360