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

[David Green via llvm-commits]

davemgreen wrote:

How do we want to handle for things like `@llvm.sadd.sat.i64` (on Arm, or i128 on 64bit systems)? With this patch they will be costed as `2 * cost(i32)`. I don't mind adding custom cost model rules if needed, but it feels like something that the generic cost model should be getting more correct.

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