[llvm] [AArch64][GlobalISel] Combine Shuffles of G_CONCAT_VECTORS (PR #87489)

[via llvm-commits]

chuongg3 wrote:

> [varargs](https://github.com/llvm/llvm-project/issues/87459) are unfortunately not here yet, but there are now several combines registered on G_SHUFFLE_VECTOR that will always fail because the input is bad.
> 
> ```
> def shuffle_vector_of_concats2 : GICombineRule<
>    (defs root:$root, build_fn_matchinfo:$matchinfo),
>    (match (G_CONCAT_VECTORS $src1, $a, $b),
>           (G_CONCAT_VECTORS $src2, $c, $d),
>           (G_SHUFFLE_VECTOR $root, $src1, $src2, $mask),
> ```
> 
> ```
> def shuffle_vector_of_concats3 : GICombineRule<
>    (defs root:$root, build_fn_matchinfo:$matchinfo),
>    (match (G_CONCAT_VECTORS $src1, $a, $b, $c),
>           (G_CONCAT_VECTORS $src2, $d, $e, $f),
>           (G_SHUFFLE_VECTOR $root, $src1, $src2, $mask),
> ```

I believe using one pattern to match all the different G_CONCAT_VECTORS sizes would allow for more generalization of arbitrary G_CONCAT_VECTORS sizes until variable length arguments are supported in tablegen.

https://github.com/llvm/llvm-project/pull/87489