[AArch64][GlobalISel] Mark v16s8 <- v8s8, v8s8 G_CONCAT_VECTOR as legal

G_CONCAT_VECTORS shows up from time to time when legalizing other instructions.

We actually import patterns for the v16s8 <- v8s8, v8s8 case so marking it
as legal gives us selection for free.

Differential Revision: https://reviews.llvm.org/D107512