This pipeline is run as part of IRGen and has access to the IRGenModule.
Passes that run as part of this pipeline can query for the IRGenModule.
We will use it for the AllocStackHoisting pass. It wants to know if a type is of
non-fixed size.
To break the cyclic dependency between IRGen -> SILOptimizer -> IRGen that would
arise from the SILPassManager having to know about the createIRGENPASS()
function IRGen passes instead of exposing this function dynamically have to add
themselves to the pass manager.
This commit adds the functionality, but does not change SILPassManager to use
it. The reason why I am doing this is so I can implement sil-opt pass bisecting
functionality in python using a tool that dumps the current pass pipelines
out. This will ensure that even in the face of changes to the pass pipelines,
everything should just work.
