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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

@jayfoad Thanks for the review. However, after some reflection and offline discussion with @nikic, we have decided to close both PRs, since the patches don't have any benefit on real-world code, and just add compile-time. A completely optimal mul might be beneficial from a testing point of view but, as the real-world benchmarks indicate, a purely academic exercise.

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