Add a `phase_accurate` option to the compiler and classify existing optimizations

[dime10]

Since we might not want to restrict ourselves to purely phase-accurate optimizations, it might be nice to add an option to the compiler that controls whether phase-inaccurate optimizations are applied as well.

We'd have to decide what the default is. Currently, the optimization referenced in this issue is phase inaccurate:

Originally posted by @dime10 in #1827