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

[Ramkumar Ramachandra via llvm-commits]

artagnon 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.

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