AFAIR from a course at university, the status quo (about five years ago) was that gcc did not implement many of the advanced optimization techniques, such as IPS (integrated pre-pass scheduling). I have not checked myself, just promulgating--albeit competitive--hearsay...
gcc and llvm don't have a technique with that name, but it sounds a little like an implementation detail to me. What is it prior to and/or integrated with?
gcc supports profile-guided optimization just fine, llvm has some code for it but I'm not sure if it's hooked up. Neither of them use iterative techniques for optimization - they're already too slow as it is for most people, anyway.
Instruction scheduling of any kind doesn't really help on x86 anyway, and register pressure is usually surprisingly good already (since temporary values are moved close to their uses when combining instructions).
I think the most important thing missing thing is rematerialization - recalculating values instead of saving them on stack would save a lot of memory loads.
