During the Game Developer's Conference 2014 this week I caught up with Jens Owen of LunarG. Among other topics, he talked about the LunarGLASS LLVM shader compiler stack. LunarGLASS has been open-source since 2010 as a "open source, long-term, highly-functional, and modular shader and kernel compiler stack." LunarGLASS isn't currently part of the mainline Mesa tree, but it continues to be worked on by LunarG employees.
You can see recent changes to LunarGLASS via their Google Code page, with the most recent work to it having been an update so it now runs on LLVM 3.4. Jens was continuing to talk up the benefits of LLVM for compiler infrastructure and optimizations at GDC 2014, while acknowledging there is a high up front cost in switching to LLVM, although some other developers (namely Intel OTC with their open-source graphics driver) continue to be against the use of LLVM.
More details on LunarGLASS can be learned from LunarGLASS.org.