nir: Handle FB fetch outputs correctly in nir_lower_io_to_temporaries.

nir: Handle FB fetch outputs correctly in nir_lower_io_to_temporaries.

This requires emitting a series of copies at the top of the program
from each output variable to the corresponding temporary.  The initial
copy can be skipped for non-framebuffer fetch outputs whose initial
value is undefined, and the final copy needs to be skipped for
read-only outputs (i.e. gl_LastFragData), since it would be illegal to
emit a store output intrinsic for it.

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