[llvm] [ValueTracking] Support horizontal vector add in computeKnownBits (PR #174410)

[Valeriy Savchenko via llvm-commits]

SavchenkoValeriy wrote:

> Moving non-trivial logic into KnownBits is generally a good idea, because that's how we reuse logic between the middle-end and back-end implementations of computeKnownBits(). (We could use the same logic for VECREDUCE_ADD.)

So, moving just that bit of logic into KnownBits would be enough, right? And what about the signature for it. I guess it should be something like `(KnownBits Arg, std::optional<unsigned> N)`?

https://github.com/llvm/llvm-project/pull/174410