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

Combines G_SHUFFLE_VECTOR whose sources comes from G_CONCAT_VECTORS into
a single G_CONCAT_VECTORS instruction.

a = G_CONCAT_VECTORS x, y, undef, undef
b = G_CONCAT_VECTORS z, undef, undef, undef
c = G_SHUFFLE_VECTORS a, b, <0, 1, 4, undef>
===>
c = G_CONCAT_VECTORS x, y, z, undef`