[llvm] [KnownBits] Add KnownBits::add and KnownBits::sub helper wrappers. (PR #99468)

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goldsteinn wrote:

I'm opposed to no option for `NUW`/`NSW`. I think this will probably become the defacto API and think this basically encourages people, especially new contributors, to forgetting/ignoring the flags.

Can you add them as default false params?

https://github.com/llvm/llvm-project/pull/99468