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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> > 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.
> 
> I also suspect that it's not optimal for high bits: I think we need some logic similar to the low-bits logic later in the function.

Quick remark: I think this is sub-optimal due to the mul_ov overflowing in many cases.

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