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

[Matt Arsenault via llvm-commits]

arsenm 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.

It's probably not the best default for custom lowering, but once something is custom, backend intervention is kind of unavoidable. If we want to change the default custom cost heuristic, that would be a bigger and separate change. 

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