This computes the address sent to the MMU separately from that sent
to the dcache. This means that the address sent to the MMU doesn't
have the delay through the lsu_sum adder, making it available earlier.
The path through the lsu_sum adder and through the MMU to the MMU
done and err outputs showed up as a critical path on some builds.
Signed-off-by: Paul Mackerras <firstname.lastname@example.org>