glsl: Move get_{column,element} to expression args.
glsl: Move get_{column,element} to expression args.

I think this makes the code more obvious by moving the declarations to
their single usage (now that we aren't using them to get at the ->type
field for expression constructors).

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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