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.

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

The node must use the mode of the floating point register class, irrespective of the mode of the loaded constant.

IncSP can only have %esp assigned.

arm, be, ia32, sparc: Do not go on with peephole optimization of IncSP, when be_peephole_IncSP_IncSP() succeeded.

The current IncSP was removed in this case and we now would operate on its predecessor IncSP.
This is wrong, e.g. when asking for currently free registers.
This corrects backend/incsp_pop.c, which was broken on ia32.
It was wrongly assumed that a certain register was free and could be used to turn an IncSP into a Pop.