Normalize Or -> Add only for two's complement modes.

This fixes opt/fehler230.c.
Merged fp-vrp into localopt.

Along the way:
- Simplified code to sharpen Cmp relations
- Do not remove unreachable code by using bit information anymore
amd64: do not use RIP relative addressing for now

Apparently it is optional on linux (just often used because the RIP
encoding is often shorter than a global encoding).