[llvm] [InstCombine] Move extends across identity shuffles. (PR #146901)

[Florian Hahn via llvm-commits]

fhahn wrote:

> 
> Seems reasonable. We can always revisit if needed.

Yep, from my experiements on AArch64, this mostly helps with code written with vector intrinsics. We can iterate/evolve the logic as other motivating cases arise

https://github.com/llvm/llvm-project/pull/146901