[llvm] [KnownBits] Add fast path for udiv by pow2 and overloads for shift ops by constant (PR #189779)

[Nikita Popov via llvm-commits]

nikic wrote:

> Also, I think to make `explicit KnownBits(unsigned BitWidth)`. This kind of implicit conversion wasn't very reliable before, but now it's even more important and may lead to confusions. WDYT? Of course in separate PR due to it may add more unrelatable changes

Sounds reasonable

https://github.com/llvm/llvm-project/pull/189779