This allows us to easily find all branches that use a label.
Since 'refs' is only tested vs zero, remove it and test for
an empty list instead.  Drop the use of bitfields, which had
been used to pack refs into a single 32-bit word.

Reviewed-by: Taylor Simpson <tsimpson@quicinc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>