[llvm] [SelectionDAG] Lowering usub.sat(a, 1) to a - (a != 0) (PR #170076)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@rez5427 I've added x86 test coverage at #170681 - you will need to merge and update the test after it lands

https://github.com/llvm/llvm-project/pull/170076