After reading your explanations, it seems reasonable to assume that a shader compiler is *very* hardware-specific, thus probably developed in-house,
A large part of a shader compiler is still the generic optimization passes. GCC has different backends for different CPUs, and different optimization passes enabled for different targets, but the meat of the optimization engine is still generic across all architectures. Same with shaders. Giving up those optimization passes could get competitors a huge advantage (though, honestly, general opinion is that ATI's drivers really suck compared to NVIDIA's, so I'm not sure there's anything ATI has that NVIDIA or even Intel is likely to be really interested in).