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

[Mircea Trofin via llvm-commits]

mtrofin wrote:

> 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.

I was thinking about that too, my worry is that a wholesale exclusion would allow anything to pass in the future (like, a branch). I realize, on one hand, that it should probably not happen in this case, but on the other hand, we're replacing a one-liner with another one-liner.

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