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

There are 2 cases: 

- either the `select`​ condition is a vector of bools, case in which we don't currently have a way to represent the per-element branch probabilities anyway;
- or the select condition is a scalar, for example from a `llvm.vector.reduce`​. We could potentially try and do more here - if the reduced vector contained conditions from other selects, for instance

In either case, IIUC, chances are the `select`​ doesn't get lowered to a branch, at least I'm not seeing any evidence of that in an internal complex application (CSFDO + ThinLTO). Seems sufficient to mark the selects are unknown (for profiled functions); since that metadata carries with it the pass name (`DEBUG_TYPE`​) that marked it as such, we can revisit this if we detect later lowerings of these selects that would have required an actual profile.



Issue #147390

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