[llvm] [PredicateInfo] Support `PredicateInfo` by adding `PHI` nodes that pick up info from created branch predicates (PR #151132)

[Luke Lau via llvm-commits]

lukel97 wrote:

> > Can you point out which parts are complex?
> > Also there's a limitation we can set to number of such nodes we can introduce, I believe it's 4/8 similar for branch/assume which already exist, can do the same for this too.
> 
> I mean the time/space complexity. Yeah the compile-time overhead can be mitigated by specifying a threshold. But the most important thing is to demonstrate its usefulness in more real-world programs...

+1, before this patch can be accepted I think it needs to show some utility in benchmarks or applications. There's some useful policy written about this in the InstCombine guide but I think it applies for most areas in LLVM: https://llvm.org/docs/InstCombineContributorGuide.html#id18

https://github.com/llvm/llvm-project/pull/151132