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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Can we keep the existing cost estimate code, but instead of returning Cost just stash it as a optional value and fallback to it if the ISD legality fails?

I'm not sure what it means for the ISD legality to fail? 

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