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

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

The implementation looks fine to me. As you say it's strictly better than the status quo.

I'd prefer to put the ad hoc tests in `unittests/Support/KnownBitsTest.cpp` since it's a more direct way of testing the KnownBits implementation than going via ValueTracking.

For future work: I suspect that what you have implemented here is not quite optimal for the high bits. We could fix that and extend #113316 to check optimality of high bits as well as low bits.

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