[llvm] [InstCombine] Improve shamt range calculation (PR #72535)

[Nikita Popov via llvm-commits]

nikic wrote:

I think there are really two independent changes here: 1. Handle unknown shift amounts and 2. use constant range. The majority of the test changes is caused by 1. Can you please reduce this PR to just the unknown shift amount handling and separate ConstantRange into a separate one?

https://github.com/llvm/llvm-project/pull/72535