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prog->nir will generate fsub opcodes, but i965 doesn't implement them. We may as well lower them at the NIR level, since it's trivial to do. Suggested by Connor Abbott. Signed-off-by: Kenneth Graunke <kenneth@whitecape.org> Reviewed-by: Connor Abbott <cwabbott0@gmail.com> Reviewed-by: Eric Anholt <eric@anholt.net>