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

[David Green via llvm-commits]

https://github.com/davemgreen commented:

The `llvm.sadd.sat.i64` Arm costs are a bit odd - I assume because `getTypeLegalizationCost(i64)` will be `{2, i32}`, and it uses the `LT.first * 2` as the `i32` is legal. The other results seem OK but that one doesn't sound right for this operation. I could add an override in the Arm backend if you think that is the best way forward - the other results look better, including the SVE versions that we don't currently have tests for.

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