AMDGPU/GlobalISel: Fix assert on 16-bit G_EXTRACT results

I consider this to be a hack, since we probably should not mark any
16-bit extract as legal, and require all extracts to be done on
multiples of 32. There are quite a few more battles to fight in the
legalizer for sub-dword vectors, so just select this for now so we can
pass OpenCL conformance without crashing.

Also fix the same assert for G_INSERTs. Unlike G_EXTRACT there's not a
trivial way to select this so just fail on it.