LLVM 2.9 will be released very soon, and Emscripten has just been updated to support it.
Emscripten has a lot of automatic tests - they take over 2 hours to run on my laptop - so I won't be running tests for LLVM 2.8 anymore (that would double the time the tests take). Until LLVM 2.9 is formally released with binary builds, you can build LLVM from svn source (the instructions on the Emscripten wiki are useful), or use LLVM 2.8 with Emscripten 0.9 (the last release of Emscripten that supports 2.8).
If you do build LLVM 2.9 and put it in a different location than 2.8 was, don't forget to update your ~/.emscripten file so it uses the version you want. Also, if you update LLVM to 2.9 and want to use llvm-gcc, you need to update that to their current svn as well.
There were not a lot of changes for Emscripten to support 2.9, so it is possible 2.8 will still work. But as mentioned above, I am not testing it, so I can't say for sure.