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

[Sam Parker via llvm-commits]

https://github.com/sparker-arm commented:

I think the BasicTTI changes make sense. But for any backends, such as AArch64, which have already made an attempt to cost the saturating intrinsics, it looks like we're now losing out.

I'm wondering if using `getArithmeticInstrCost` would help in the general sense even though, from a brief look, the AArch64 backend wouldn't handle it properly there either.

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