[llvm] [Instcombine] Lower to explicit subtraction + unsigned comparison (PR #170896)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

We do consider usub.with.overflow non-canonical, so in principle this is fine (if we're willing to overlook the multi-use undef). I'm a bit concerned that we may not always recover this in the backend, which is more problematic when we originally started from an overflow builtin.

https://github.com/llvm/llvm-project/pull/170896