I doubt it. Such a replacement would be a huge effort to write (including integrating it with the existing VM) and maintain, and there are also political factors. Despite those, I don't think it's out of the question that it will happen some day, especially as Chrome already has a copy of LLVM, though I do think it's less relevant than the other stuff I mentioned.
Sure it definitely would be a lot to implement _now_, but llvm was released roughly 5 years before v8. The v8 team totally could have chosen it for a compiler back end (code generation). My observation was simply that they didn't, and I believe there probably was a reason for it (though I can't speak to it).
In hindsight, it probably would've made more sense for Google to utilize llvm as a compiler IR in v8 than as a virtual machine as in PNaCl. [1]
edit: azakai has a better answer :)