Now register requirements always have have a register class.
This is more consistent and removes some null pointer checks.

- Rename other_different field to must_be_different
- Remove arch_register_req_type_must_be_different, it's enough to see
  if the must_be_different field is not zero.

- Rename other_same field to should_be_same.
- Remove arch_register_req_type_other_same, it's enough to see if the
  should_be_same field is not zero.

When there is a must_be_different constraint, it must be handled.
The mode of the value should not influence this.
This was some very old code.

This should hold now that no more empty spill infos are generated for Phi arguments.

This removes the last attributes from struct arch_env_t.

be: Move the info about the register classes and registers struct the arch_env_t into struct arch_isa_if_t.

Just use the selected isa interface directly.

In particular create Keeps for mode_T nodes in one go instead of adding one Proj to the Keep after another.

The only user should be an arch_add_irn_flags() instead.
It was harmless though, because no other flags were set.

Rematerialisations involving multiple nodes and Projs (e.g. Proj->Load) got miscounted.
Now a rematerialization always counts as 1.

The node was just created, so it is not scheduled.