This fixes opt/ifconv10.c and opt/ifconv12.c.

It is the default for width != 1 now.
Work on this never was finished, so it probably does not work anyway.

This way all the backends do not have to filter it by themselves.

In particular do not stop the search for a suitable node, when a non-mode_is_data() node is encountered.

The verifiers for the checked nodes are at least as strict as these assert()s.

This fixes backend/spill_phi2.c.

The same check is just a few lines above.

With base the displacement may be a single byte.

The epilogue is added late, in particular after scheduling.
Therefore it is not necessary to move a mis-scheduled 'restore' directly before a 'return' anymore.