It’s just meant to reference a person (a “writer”) who writes some transformation (a “pass”) for the FX IR. E.g. relevant wiki page on a compiler that does multiple passes:

  

      en.wikipedia.org
  

  
    

Multi-pass compiler

A multi-pass compiler is a type of compiler that processes the source code or abstract syntax tree of a program several times. This is in contrast to a one-pass compiler, which traverses the program only once. Each pass takes the result of the previous pass as the input, and creates an intermediate output. In this way, the (intermediate) code is improved pass by pass, until the final pass produces the final code.
 Multi-pass compilers are sometimes called wide compilers,[citation needed] referrin...