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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> If you multiply with twice the bit width, it will never overflow.

Right. I _think_ I can work on a follow-up doing the mul_ov with twice the bitwidth, and make the high bits exhaustive. Will investigate after this patch lands.

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