Have a look at pnacl - LLVM executables in NACL sandbox:

  http://nativeclient.googlecode.com/svn/data/site/pnacl.pdf

  http://www.chromium.org/nativeclient/pnacl

Shipping LLVM bitcode is a bad idea. The bitcode format is constantly changing from revision to revision, its JIT is huge and slow so you'd have to rewrite a lot of it, .bc files are enormous, the bitcode format is non-portable, the type system is missing a bunch of stuff like unions, etc.

And modern JavaScript engines largely make it fast enough anyway.