In general, almost all of it. If you haven't laid out your data to make it accessible to SIMD instructions, at best the program will get a modest speedup by using most of the instructions rearranging data to be SIMD-compatible.

By far the simplest way of making the correct data-layout is to use the intrinsics manually, so that you discover any issues that arise.

A step further is to change the program logic to better take advantage of SIMD-instructions, for instance Fabian Giesen has written a great deal on making compression code that splices the jobs in creative ways in order to utilize SIMD-instructions.