[gsg]

Specialising LLVM for dynamic compilation might well come at the expense of LLVM's strengths as an AoT compiler, and it would probably not be easy in any case.

In addition to that, dedicated implementations can take various shortcuts to make their job easier - there are some nice examples given in the link. LuaJIT is example of a compiler project that benefits from being heavily specialised to a particular job, to remarkable effect.