glsl: Always copy the index when cloning a variable.
glsl: Always copy the index when cloning a variable.

The index is also used for GL_ARB_blend_func_extended.  Cloning in
i965 was dropping a non-ARB_explicit_attrib_location index.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

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