gallivm: optimize SoA AoS fallback fetch path a little

gallivm: optimize SoA AoS fallback fetch path a little

We should do transpose, not extract/insert, at least with "sufficient" amount
of channels (for 4 channels, extract/insert shuffles generated otherwise look
truly terrifying). Albeit we shouldn't fallback to that so often in any case.
v2: ditch the extract/insert path, not worth keeping (we're going to avoid
hitting the fallback that often with future patches).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>