[llvm] [InstCombine] Set !prof metadata on Selects identified by add.ll test (PR #158743)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

> I'd like this metadata to only be generated if profcheck is enabled. Otherwise it just adds noise to the IR.

We could gate inserting `unknown` on whether the function has an entry count that's also not 0. That way, tests, or code without profile info to begin with, aren't affected; tests under profcheck have it, as intended (because we also set a nonzero function entrycount); and, importantly, production builds (with profile info) have it, too, for non-cold functions, and we can start using ORE for an additional layer of diagnostics.

wdyt?

https://github.com/llvm/llvm-project/pull/158743