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

[via llvm-commits]

goldsteinn wrote:

> > How does this compare to Jay's impl [#86671 (comment)](https://github.com/llvm/llvm-project/issues/86671#issuecomment-2023064384)?
> 
> My patch makes KnownBits::mul strictly better for the high bits: I don't touch the low bits. Jay's implementation attempts to rewrite mul completely, at the cost of compile time.

Strictly better than current or jays impl?

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