AMDGPU/GlobalISel: Don't use vector G_EXTRACT in arg lowering

Create a wider source vector, and unmerge with dead defs like the
legalizer. The legalization handling for G_EXTRACT is incomplete, and
it's preferrable to keep everything in 32-bit pieces.

We should probably start moving these functions into utils, since we
have a growing number of places that do almost the same thing.