[llvm] [LLVM][SVE][CodeGen] Fix incorrect isel for signed saturating instructions. (PR #88136)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

Given it involved editing largely the same code I've included the changes to ensure optimal code generation. Please let me know if you prefer the bug fix to land separately.

https://github.com/llvm/llvm-project/pull/88136