[AArch64][GlobalISel] Widen G_ADD/G_MUL/G_OR/... element types if size < 8b

Widen element types of vector arguments of G_ADD, G_SUB, G_MUL, G_AND,
G_OR, G_XOR to the minumum supported size, in order to support vectors
of narrow types.

Differential Revision: https://reviews.llvm.org/D143100