[AArch64][GlobalISel] Widen (<2 x s16> = G_BUILD_VECTOR) to <2 x s32>.

We don't support this as a argument or return type, it's always promoted to <2 x s32>.

Performing the widening prevents us from having selection failures due to unsupported
extends.

Fixes https://github.com/llvm/llvm-project/issues/58274