First it is unclear what is a good way to deconstruct a program into extended
basic blocks, also none of our optimizations works on extended basic
blocks.

This call initializes the node_nr with a random value on startup. This
lets you test wether the compile behaves deterministically regardless of
node numbers (which isn't always the case right now).

This makes the produced assembler independent from fragile node numbers.
It's also a little bit easier to read.

Also factor out code that emits the label and comment at the beginning
of a basic block.

It probably was true when the start block had itself as predecessor.

This was used as a hack to mark unreachable code by some people, but it
is prefered to simply use a new Block without predecessors, or set NULL
if you have to.

Change tests for ir_visibility_external, which should really test for entity_has_definition(), since the meaning of ir_visibility_external changed.

For global variables defined in the current translation unit this restores that
- they are not accessed via trampolines when compiling PIC,
- they are not skipped when emitting debug info.