[llvm] Modify call to Knownbits::computeForAddSub when second operand is known to be negative (PR #210954)

[Nikita Popov via llvm-commits]

https://github.com/nikic requested changes to this pull request.

Fix is incorrect, and most likely trying to work around UB in user code.

https://github.com/llvm/llvm-project/pull/210954