[llvm] ValueTracking/test: cover known bits of mul (PR #113050)

[via llvm-commits]

goldsteinn wrote:

Typically its nicer to have these as part of the PR, otherwise its hard to see if they are in fact appropriate.
But LGTM.

Also regarding the commit messages (particularly the `[<Component>]` practice, found the reference: https://llvm.org/docs/DeveloperPolicy.html#id18), can you start doing it?

https://github.com/llvm/llvm-project/pull/113050