gallivm: fix indirect addressing of inputs

gallivm: fix indirect addressing of inputs

We weren't adding the soa offsets when constructing the indices
for the gather functions. That meant that we were always returning
the data in the first element.
(Copied straight from the same fix for temps.)
While here fix up a couple of broken comments in the fetch functions,
plus don't name a straight float type float4 which is just confusing.

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