i965/vec4: fix splitting of interleaved attributes

i965/vec4: fix splitting of interleaved attributes

When we split an instruction that reads an uniform value
(vstride 0) we need to respect the vstride on the second
half of the instruction (that is, the second half should
read the same region as the first).

We were doing this already, but we didn't account for
stages that have interleaved input attributes which also
have a vstride of 0 and need the same treatment.

Fixes the following on Haswell:
KHR-GL45.enhanced_layouts.varying_locations
KHR-GL45.enhanced_layouts.varying_array_locations
KHR-GL45.enhanced_layouts.varying_structure_locations

Reviewed-by: Matt Turner <mattst88@gmail.com>
Acked-by: Andres Gomez <agomez@igalia.com>