[llvm] KnownBits: refine high-bits of mul in signed case (PR #113051)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> I'm generally in favor of this, but I'm also in favors of jays impl getting in unless the compile time data is really bad.

I think looping through bits is unnecessary, as low-bits are already optimal, and we need something like that for high bits: we can work on a follow-up.

https://github.com/llvm/llvm-project/pull/113051