Support prefix padding for alignment purposes (Relaxable instructions only)

Now that D75203 has landed and baked for a few days, extend the basic approach to prefix padding as well. The patch itself is fairly straight forward.

For the moment, this patch adds the functional support and some basic testing there of, but defaults to not enabling prefix padding. I want to be able to phrase a separate patch which adds the target specific reasoning and test it cleanly. I haven't decided whether I want to common it with the nop logic or not.

Differential Revision: https://reviews.llvm.org/D75300