[llvm] [X86] combineCONCAT_VECTORS - fold concat(extract_subvector(X,0),extract_subvector(Y,0)) --> shuffle(X,Y) (PR #135985)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Agreed, I will have another look at seeing whether we can get the existing generic folds to fire for this one

https://github.com/llvm/llvm-project/pull/135985