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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> And I see now that your follow-up PR was already tested in [dtcxzyw/llvm-opt-benchmark#1591](https://github.com/dtcxzyw/llvm-opt-benchmark/pull/1591), and does not seem to provide benefit on real code.

True, and that came as a very unfortunate surprise, as I spent quite a lot of time on these PRs :(

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