A Javascript JIT doesn't have to make the check, because it can simply decide that all memory blocks, and all copies, occur on word boundaries.
One of the problems with optimising C is that you have to assume (in simple terms, obviously the full story is more complicated) whenever a user's function is called, then all of memory might have changed. Even if you have a pointer to a const int, maybe in another part of the code there is another pointer to that int which isn't const, so you have to assume it might have changed.
In a language with different semantics, the optimisers can have a much easier job seeing what is going on, and know what can effect what else. This is the reason that Fortran compilers can often be seen beating C compilers.
One of the problems with optimising C is that you have to assume (in simple terms, obviously the full story is more complicated) whenever a user's function is called, then all of memory might have changed. Even if you have a pointer to a const int, maybe in another part of the code there is another pointer to that int which isn't const, so you have to assume it might have changed.
In a language with different semantics, the optimisers can have a much easier job seeing what is going on, and know what can effect what else. This is the reason that Fortran compilers can often be seen beating C compilers.