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

Now add_pressure[] is copied.

Actually, I just want to trigger the buildbot and see if my fix worked.

Avoid using the op_pin_state enum as that contains confusing additional
entries that are not valid on a node. The only remaining state for a
node is pinned/floating which we can simply represent with a 0/1 value.

I can't explain why the copy was created there, the comment explaining
it makes no sense.

This case cannot happen anymore nowadays, instead we have some code that
turns CopyKeeps into Keeps after the SSA reconstruction there.

This adds two builtin kinds, ir_bk_va_start and ir_bk_va_arg. va_arg Builtins
are lowered in the lower_builtins pass with a function provided by the backend,
whereby most architectures can use be_default_lower_va_args. va_start Builtins
are lowered in the backend.

Note: This commit breaks all backends except for IA32. SPARC and AMD64 are
implemented in later commits. The ARM backend previously miscompiled variadic
functions, and will now throw a proper error.

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

Do not derive another mode from the mode of the value -- they are the same anyway.
I could trace this code back to when backend did not always use register wide modes for values.

It is now done generically in dump_backend_info_hook().

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

They all have a single memory result.

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