decode1: Avoid overriding fields of v.decode in decode1

decode1: Avoid overriding fields of v.decode in decode1

In the cases where we need to override the values from the decode ROMs,
we now do that overriding after the clock edge (eating into decode2's
cycle) rather than before.  This helps timing a little.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>