nir: recombine nir_op_*sub when lower_sub = false

nir: recombine nir_op_*sub when lower_sub = false

There are some optimizations which are only implemented for additions
and some optimizations which assume that subtractions have been lowered.
By lowering all subtractions first and later recombine for backends
which prefer this option, we don't have to implement them twice.

This patch also moves lower_negate to nir_opt_algebraic_late() to enable
these optimizations for backends which make use of it.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>