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

[via llvm-commits]

goldsteinn wrote:

> > Still think a test where this actually comes up in CodeGen somewhere would be nice, but the code all looks good.
> 
> @goldsteinn I agree, this would be nice to have, but I wonder if it's worth blocking this PR on. Based on what I understand - this is my first contribution to LLVM - , we're still missing compiler intrinsics and Target lowering code, before we can test the compiler optimization on LLIR, which would all be out of scope of the ticket this is addressing. I've added regression tests ensuring that the KnownBits calculation works, so I don't think we're particularly lacking test coverage here. What are your concerns specifically?

Sorry missed this, I didn't mean for the concern to be blocking this.

If its not really doable that okay and this change LGTM.

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