[llvm] TTI: Fix special casing vectorization costs of saturating add/sub (PR #97463)

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

I think this unfortunately also sets the precedent that some nodes that are _not_ legal/custom also need to be handled by the target.

Having said that the Arm/AArch64 costs look OK to me. LGTM

https://github.com/llvm/llvm-project/pull/97463