These are lowered, matching SDAG behaviour. (See llvm/test/CodeGen/AArch64/ssub_sat.ll and llvm/test/CodeGen/AArch64/sadd_sat.ll)
These fall back ~159 times on a build of clang with GISel enabled.
Abort is disabled here because we can't clamp these instructions yet. The s88 test requires widening to s128 then clamping back down. I think this is consistent with what SDAG does: https://godbolt.org/z/cn3fWGvqx