Revert "glsl: Skip processing the first function's body in do_dead_functions()."

Revert "glsl: Skip processing the first function's body in do_dead_functions()."

opt_dead_functions contained a shortcut to skip processing the first
function's body, based on the assumption that IR functions are
topologically sorted, with callees always coming before their callers
(therefore the first function cannot contain any calls).

This assumption turns out not to be true in general.  For example, the
following code snippet gets translated to IR that violates this
assumption:

    void f();
    void g();
    void f() { g(); }
    void g() { ... }

In practice, the shortcut didn't cause bugs because of a coincidence
of the circumstances in which opt_dead_functions is called:

(a) we do inlining right before dead function elimination, and
    inlining (when successful) eliminates all calls.

(b) for user-defined functions, inlining is always successful, because
    previous optimization passes (during compilation) have reduced
    them to a form that is eligible for inlining.

(c) the function that appears first in the IR can't possibly call a
    built-in function, because built-in functions are always emitted
    before the function that calls them.

It seems unnecessarily fragile to have opt_dead_functions depend on
these coincidences.  And the next patch in this series will break (c).
So I'm reverting the shortcut.  The consequence will be a slight
increase in link time for complex shaders.

This reverts commit c75427f4c8767e131e5fb3de44fbc9d904cb992d.

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