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This resolves problems with hidden dependencies during code selection, which cause dependency cycles and therefore fixes backend/scheduled.c.
Also it grants the code selection slightly more freedom by not arbitrarily restricting the order by stack dependencies.

This removes the special cases in amd64_has_addr_attr(), which corrected for the wrong op_mode.

This fixes backend/amd64_sub2.c.

All backends had identical implementations.
Now rematerialization works out of the box for new backends.

All backends have similar empty Start nodes, which just provide registers.
So having one generic backend Start node to share some infrastructure is sensible.

bescripts: Automatically determine the mode of nodes by default and add the option "first" to use the mode of the first output requirement.

They only have one result.
This also resolves the harmless typo, that they used each others Proj enums.

It pops a value from the stack, so this memory access should be represented in the graph.

We never used the distinction which flags get modified, so just simplify the specification a bit.

Handle this similar to the ia32 backend.

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.