- 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.

We can easily identify the outputs producing a stack pointer by looking
if the stack pointer register is assigned. Unfortunately this also
needed a new mechanism to tell the verifier that some registers are not
in SSA form (all the time) which needs a few extra lines in each
backend.

Get the obstack from the given start node.

Use a flexible array, so we can iterate over the linked list only once instead of twice.

This fixes gcc/pr43784.c.

This fixes several test on ARM.

This fixes opt/reassoc031.c.

This gives the finish phase enough room to copy registers if
necessary.

Instead of mvn + and we get a single bic now.

The case for <= 255 just below includes 0 and gives the same result.

The spiller crashes on some unoptimized Phi nodes.

Fixes test backend/amd64_source_am.c