[AArch64][GlobalISel] Fix manual selection for v4s16 and v8s8 G_DUP

The manual G_DUP selection code would produce DUPv16i8 for v8s8s and DUPv8i16
for v4s16.

This adds the missing cases to the manual selection code, and makes it return
false when there is an unexpected size.

Update select-dup.mir to reflect the change.

Differential Revision: https://reviews.llvm.org/D97240