Intel are in a perpetual war with another vendor with the same instruction set for which some of their optimizations are generally applicable, to the point that ICC used to intentionally cripple AMD hardware: http://www.agner.org/optimize/blog/read.php?i=49#49

Thus there's a disincentive for Intel to release their optimizer's tricks: not only are at least some percentage of the optimizations applicable to their competitor's microarchitecture implementing the same ISA, but they probably reveal various Intel CPU internals that Intel consider trade secrets (similar to the argument against open-sourcing 3D drivers and shader compilers).

Mill is not going to be locked into a bitter head-to-head battle with someone else trying to implement the same ISA better (at least not for a long time), so there's no incentive for them to hide their CPU's internal optimizations and no competition for which compiler optimizations could be generally applicable.

I always assumed that the parts of icc that Intel want to keep secret aren't in the backend, they're in the optimisation phases, which could be ported to compilers for other architectures.