glsl: Only create one ir_function for a given name.

Piglit's spec/glsl-1.10/linker/override-builtin-{const,uniform}-05 tests
do the following:

1. Call abs(float) - a built-in function.
2. Create a user-defined replacement for abs(float).
3. Call abs(float) again - now the user function.

At step 1, we created an ir_function which included the built-in
signature, added it to the symbol table, and emitted it into the IR
stream.

Then, when processing the function definition at step 2, we'd see that
there was already an ir_function.  But, since there were no user-defined
functions, we skipped over a bunch of code, and ended up creating a
second one.  This new ir_function shadowed the original in the symbol
table, but both ended up in the IR stream.

This results in an awkward situation where searching for an ir_function
via the symbol table, a forward linked list walk, and a reverse linked
list walk may return different ir_functions.  This seems undesirable.

This patch instead re-uses the existing ir_function, putting both
built-in and user-defined signatures in the same one.  The previous
patch's additional filtering ensures everything continues working.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>

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