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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> in which case why not just drop the `!TLI->isOperationExpand(ISD, LT.second)` case entirely?

That's a broader scoped change than this patch, which is just trying to send the intrinsic down the normal ISD path. 

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