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Add callbacks on functions that really need them instead.

When creating an entity for a floatingpoint constant, first try to reduce the size, then check, whether it already exists.
This way only a single entity is created for e.g. a double and a float 1.0.

* Also only use the necessary number of bits.
* Improve its comment.

* Classify ',' as "no support" instead of "invalid".
* Ignore '!' and '?'.

* GCC does this, too.
* This is sensible, because they apply to all alternatives of a multi-alternative constraint.
* This simplifies the implementation and makes some flags obsolete.

This also renders get_irg_n_anchors() obsolete, so remove it.

This is useful for backend development.