[AArch64][GlobalISel] Add a post-legalize combine for lowering vector-immediate G_ASHR/G_LSHR.

In order to select the immediate forms using the imported patterns, we need to
lower them into new G_VASHR/G_VLSHR target generic ops. Add a combine to do this
matching build_vector of constant operands.

With this, we get selection for free.