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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> For the future, can you please not create separate PRs for test additions and instead include them as a separate commit? Otherwise we cannot test the impact on llvm-opt-benchmark.

The test addition is included as the first commit in this PR, no?

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