[llvm] add narrowExtractedVectorUnaryOp to simplify cast nodes (PR #87977)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> > Yes, you're right - we're getting stopped by legality tests as we have poor src/dst legality tests for cast ops.
> 
> I see, is it something that can be worked on ?

It isn't straightforward - cast ops require legality checks on the src/dst type pairs which isn't stored.

https://github.com/llvm/llvm-project/pull/87977