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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> or any backends, such as AArch64, which have already made an attempt to cost the saturating intrinsics, it looks like we're now losing out.

How is this attempting to cost these? This was just adding up the cost of the assumed expansion. I don't think any of this code here adding up a hypothetical expansions is reasonable to have as a default 

> 
> 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.

I have no idea what's going on with the soup of TTI cost functions. It's like there are 3-10 competing APIs intermixed 

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