Refactor preprocessing pass registration (#2468)

Refactor preprocessing pass registration (#2468)

This commit refactors how preprocessing pass registration works,
inspired by LLVM's approach [0]. The basic idea is that every
preprocessing pass declares a static variable of type `RegisterPass` in
its source file that registers the pass with the
`PreprocessingPassRegistry` when starting the program. The registry is a
singleton that keeps track of all the available passes and allows other
code to create instances of the passes (note: previously the registry
itself was owning the passes but this is no longer the case). One of the
advantages of this solution is that we have a list of available passes
directly at the beginning of the program, which is useful for example
when parsing options.

As a side effect, this commit also fixes the SortInference pass, which
was expecting arguments other than the preprocessing pass context in its
constructor.

This commit is required for fixing dumping pre/post preprocessing
passes. It is also the ground work for allowing the user to specify a
preprocessing pipeline using command-line arguments.

[0] https://llvm.org/docs/WritingAnLLVMPass.html