It is still a useful distinction. Programming languages are complex enough to understand. It is useful to have a base of 'this is the simplest description of how programs will work' that is as simple as possible. Then, a separate set of 'and here is a separate description of its performance characteristics; nothing in this portion should be understood to change the defined behavior of any program".
Even if that second description is stable and part of the guarantees you make, keeping it seperate is still incredibly useful from a user perspective.
From an implementation perspective, there is also a useful distinction. Optimizations take a valid representation, and turn it into a different valid representation of the same type that shares all defined behavior. This is a fairly different operation than compilation, which converts between representations. In particular, for the compilation step, you typically have only one compilation function for a given pair of representations; and if you have multiple, you select one ahead of time. For optimizations, each representation has a set of optimization functions, and you need to decided what order to apply them and how many times to do so. Compilation functions, for their part, need to deal with every difference between the two representations, whereas optimization functions get to ignore everything except the part they care about.
Even if that second description is stable and part of the guarantees you make, keeping it seperate is still incredibly useful from a user perspective.
From an implementation perspective, there is also a useful distinction. Optimizations take a valid representation, and turn it into a different valid representation of the same type that shares all defined behavior. This is a fairly different operation than compilation, which converts between representations. In particular, for the compilation step, you typically have only one compilation function for a given pair of representations; and if you have multiple, you select one ahead of time. For optimizations, each representation has a set of optimization functions, and you need to decided what order to apply them and how many times to do so. Compilation functions, for their part, need to deal with every difference between the two representations, whereas optimization functions get to ignore everything except the part they care about.