[llvm] [slp][profcheck] Mark `select`s as having unknown profile (PR #162960)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

This generally LGTM, though I do wonder whether vector selects shouldn't just be excluded wholesale from profcheck? Vector selects are not going to be converted back to branches.

https://github.com/llvm/llvm-project/pull/162960