glsl: Don't allow array splitting on function arguments.

glsl: Don't allow array splitting on function arguments.

This is the reason the declaration member existed in the reference
visitor, but I didn't copy the code from structure splitting that
avoided setting it.

This wasn't currently a problem, because we don't allow splitting of
in/out variables.  But that would be nice to change some day.

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