You're using a very specific definition of backdoor there. I can think of others that at least seem plausible to introduce at the compiler level.

Timing attacks for instance, could plausibly fall out of optimizations that terminate a loop when it's clear that the value being computed won't change (e.g. is false and is repeatedly getting anded with things). This would probably be even worse for power consumption or other side channels. For a potentially easier to measure side channel, you might try introducing some state-dependent delay (short and caused by something like that loop optimization) in a bit of code preceding a packet send.

Alternately, introducing (via incorrect optimizations) the right kinds of buffer overruns or race conditions that corrupt a pointer just right, could get you a nice stack smashing exploit with certain (very abnormal) inputs -- and remote access to the machine in question.