[llvm] [DAGCombiner] Add support for scalarising extracts of a vector setcc (PR #118823)

[David Sherwood via llvm-commits]

david-arm wrote:

Yet another attempt to reland https://github.com/llvm/llvm-project/pull/117566. The last post-commit failure exposed an existing issue where we were missing a `ExpandIntRes_SETCC` to handle i128 result types. I've added a new test for this, but it can only be exposed once the new DAG combine kicks in.

https://github.com/llvm/llvm-project/pull/118823