[llvm] [Support] Add KnownBits::computeForSubBorrow (PR #67788)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Happy for this to go in as is (with the UADDO_CARRY -> USUBO_CARRY typo fix), this is still my preferred option.

But also happy if there's still resistance to drop the SelectionDAG changes and we just get the KnownBits helper added for now (still with exhaustive tests). 

https://github.com/llvm/llvm-project/pull/67788