[llvm] [ValueTracking] Extend known bits of `mul` with self and constant (PR #81892)

[Nikita Popov via llvm-commits]

nikic wrote:

TBH, I am very skeptical about fixing these reassociation issues by introducing hacks for specific cases. I've seen a few variants of this (though, I believe, always only in constructed examples, never motivated by real-world need) and if we want to fix them, we should probably fix them by reassociating in the other direction in InstCombine.

https://github.com/llvm/llvm-project/pull/81892