While it makes little sense for the end product to have a group
containing only a single pattern, avoiding this case within an
incremental patch set is troublesome.

Because this is expected to be a transient condition, do not
bother "optimizing" this case, e.g. by folding away the group.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>