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

[Sam Parker via llvm-commits]

sparker-arm wrote:

> I have no idea what's going on with the soup of TTI cost functions.

Fair! The glories of mangling IR instructions, intrinsic calls and ISD nodes all in one, multi-layered, mess :)

> For more precise costs the target can override getIntrinsicInstrCost.

I'm not sure I follow why intrinsics would/should be more precise? In this case, doesn't the intrinsic map directly to the ISD node?

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