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This is a bigger rewrite of stack frame handling in the backend:

- Do the stack frame type layout late, after all the spill slots have
  been created. This way we can avoid (and remove) the brittle
  frame_alloc_area().
- Standardize the meaning of stack entity offsets. After stack frame type
  layout they are relative to the stack pointer position at the beginning
  of the function.  It is all in one type now, no splitting into "arg",
  "between" and "stack_frame" type anymore.
- Generalize the stack pointer simulation code to work with a single
  callback. Represent stack state as current offset and align_padding
  number. Now that callbacks can access and modify them both we do not
  need custom code in the sparc backend anymore.
- Remove alignment specification on IncSP, only keep a ignore_align flag
  used for the last IncSP that has to remove all existing sp adjustments
  and may end up on a misaligned SP on ia32/amd64.
- Align stack by default on ia32+amd64 backends.

They are not used yet as we need an adapted layout algorithm for them.

The finish phase would panic already, when the matching constraint of cas is not fulfillable.

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.