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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I've no objections to this if the ARM guys are OK with it - we're setting a precedent that custom costs should be set in the TTI

I don't see how this is setting a precedent. This is normalizing what happened for most opcodes, other than this set that didn't map the intrinsic to the ISD opcode. Conceptually custom would always require custom handling for an accurate result 


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