Find instructions in all shaders that are not contained in a function
(i.e., initializers for global variables). "Move" these instructions
to the top of the main function in the linked shader. As a
side-effect, many global variables will also be copied into the linked
shader.
This currently involves an ugly hack so that every link doesn't result
in all the built-in functions showing up as multiply defined. As soon
as the built-in functions are stored in a separate compilation unit,
ir_function_signature::is_built_in can be removed.
Uses of the bits for allocation are offset by 16, and
VERT_BIT_GENERIC0 already has the 16 offset. As a result, it was
preventing the wrong thing from being allocated.
