decode1: Take an extra cycle for predicted branch redirects

decode1: Take an extra cycle for predicted branch redirects

This does the addition of NIA plus the branch offset from the
instruction after a clock edge, in order to ease timing, as the path
from the icache RAM through the adder in decode1 to the NIA register
in fetch1 was showing up as a critical path.

This adds one extra cycle of latency when redirecting fetch because of
a predicted-taken branch.

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