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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > I thought this already landed?
> 
> The patch was reverted because it exposed a bug within legalisation (expansion) of i128 based setcc operations.

Should have the proper git revert with the reapply in the message 

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