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

[Phoebe Wang via llvm-commits]

phoebewang wrote:

I checked several tests, it seems it only happens when `trunc <8 x i64> %x to <8 x i32>` on 256-bit target. I wonder what's the reason `combineConcatVectorOfExtracts` doesn't handle it (I assume it's exactly the case the function want to handle), and why there's no problem in 256 -> 128 trunc on 128-bit target.

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