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

[Alan Zhao via llvm-commits]

alanzhao1 wrote:

> > > That also sounds reasonable to me. I assume checking for a non-zero function entry count is not particularly expensive?
> > 
> > 
> > It's fetching a function metadata, and should't be expensive. We can also cache this over the whole pass rather than checking more locally.
> 
> A cache is probably not needed, so let's start without it.
> 
> It would be nice to have a helper for this though, I'd expect this to be a rather common pattern for InstCombine in particular, so if we could just replace `SelectInst::Create()` with `createSelectWithUnknownBranchWeights()` that would be nice.

Done. I created the function `setExplicitlyUnknownBranchWeightsIfProfiled(...)` which takes an instruction and adds unknown branch weights if the enclosing function has a nonzero entry count.

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