glsl: Don't generate misleading debug names when packing gs inputs.

Previously, when packing geometry shader input varyings like this:

    in float foo[3];
    in float bar[3];

lower_packed_varyings would declare a packed varying like this:

    (declare (shader_in flat) (array ivec4 3) packed:foo[0],bar[0])

That's confusing, since the packed varying acutally stores all three
values of foo and all three values of bar.

This patch causes it to generate the more sensible declaration:

    (declare (shader_in flat) (array ivec4 3) packed:foo,bar)

Note that there should be no functional change for users of geometry
shaders, since the packed name is only used for generating debug
output.  But this should reduce confusion when using INTEL_DEBUG=gs.