Landed today in Mesa Git master were several "r600g/llvm" commits from AMD's Tom Stellard. While the LLVM back-end for the R600/SI support was actually merged into mainline Mesa earlier this month when the RadeonSI Gallium3D driver was merged, only today is it being hooked up to be made available for R600g use.
Ultimately the R600 LLVM back-end is expected to be merged into the LLVM code-base, but that hasn't happened yet (it was called for inclusion into LLVM last month, but hasn't been pulled by LLVM developers) and it's too late to be moved into LLVM 3.1. As a result, for now the R600 LLVM back-end is living in Mesa.
This LLVM back-end for R600g has been one of the items that AMD's open-source team has been working on for several months. In early December was when the R600g LLVM back-end was published. This LLVM implementation is needed for OpenCL on Radeon hardware, among other purposes.
Thanks to today's R600g LLVM commits (particularly this one), Mesa can now be built with --enable-r600-llvm-compiler to enable the LLVM shader compiler. With the support built-in, use of the LLVM shader compiler for R600g is still optional via toggling the R600_USE_LLVM environment variable.