[llvm] [AArch64] treat `@llvm.ssub.sat` the same as `@llvm.aarch64.neon.sqsub` (PR #140454)

[David Green via llvm-commits]

davemgreen wrote:

> Would it be possible to entirely remove these neon intrinsics and auto-upgrade them to the generic ones?

For other intrinsics (abs, min/max) that is a good way to go. For these though, the instructions technically set a Q flag on saturation that the users might try and use. That is not something we support at the moment (we don't model the sideeffects), but having the intrinsics present allows us to pivot if we need to in the future.

https://github.com/llvm/llvm-project/pull/140454