AMDGPU/GlobalISel: Fix some legalization of < dword vector stores

AMDGPU/GlobalISel: Fix some legalization of < dword vector stores

This avoids many instances of failing to legalize a vector truncstore
of <4 x s8> to 2 bytes. We don't perfectly handle every truncstore
yet, largely because the given set of legalization actions can't
actually differentiate between changing the result type and changing
the memory type.