Unify three copies of this code from different
branch types. Fix the case when npc == DYNAMIC_PC,
i.e. a branch within a delay slot.
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>