[compiler-rt] [Support] Add KnownBits::computeForSubBorrow (PR #67788)

[Christian Kissig via llvm-commits]

christiankissig wrote:

> It seems like this PR is ready to land. Any reason why it has not been integrated @christiankissig ?

The PR had not been merged to add a CodeGen regression test as I said I would add in https://github.com/llvm/llvm-project/pull/67788#issuecomment-1741238270 . Having looked into adding one, I realize this may need more work, compiler intrinsics for add.sub with carry. This is taking too long and may exceed the scope of the issue, so will break out into a separate piece.

I've now made the final requested changes, and this PR is ready to merge.

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