Constexpr guaranteed no runtime init in addition to const semantics.
Moving all opt/ to constexpr.
Moving all compile-unit statics to anonymous namespaces to uniformize
the method used (anonymous namespace vs static has the same behavior
here AFAIK).
Signed-off-by: Nathan Gauër <brioche@google.com>
Fix code that is traversing def-use user structure at the same time
that it is changing it. This is dicey at best and error prone at worst.
This was uncovered making a change to the id_to_user representation.