glsl: When linking, emit functions at the tail of the final linked program.

When link_functions.cpp adds a new function to the final linked
program, it needs to add it after any global variable declarations
that the function refers to, otherwise the IR will be invalid (because
variable declarations must occur before variable accesses).  The
easiest way to do that is to have the linker emit functions to the
tail of the final linked program.

The linker used to emit functions to the head of the final linked
program, in an effort to keep callees sorted before their callers.
However, this was not reliable: it didn't work for functions declared
or defined in the same compilation unit as main, for diamond-shaped
patterns in the call graph, or for some obscure cases involving
overloaded functions.  And no code currently relies on this sort
order.

No Piglit regressions with i965 Ironlake.

Reviewed-by: Kenneth Graunke <[email protected]>

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