The Radeon LLVM back-end, which is more than 19,000 lines of code, was dropped from Mesa with this commit on Friday.
Dropping the back-end happened because the R600 back-end was merged into upstream LLVM. Unfortunately the upstreaming of this AMD GPU back-end was done too late for the LLVM 3.2 release that happened in December and so it will not be until LLVM 3.3 later this year where we will see the R600 back-end. As a result, there's no external release of LLVM yet with this back-end while it's already been removed from Mesa.
For the next Mesa release (Mesa 9.1 or Mesa 10.0), Tom Stellard of AMD recommends using his third-party LLVM repository that has his latest Radeon R600 back-end improvements. For the release after that (the Mesa release of H2'2013), by then there should be a stable LLVM release with this properly supported back-end.
The AMD R600 GPU back-end to LLVM is needed if using the Radeon LLVM shader compiler and wanting to use Gallium3D OpenCL support with modern Radeon GPUs.
A follow-up commit from Friday allows for LLVM 3.2 and newer to be used in conjunction with the latest Mesa Git master.