Expose module simplification pipeline and extension points with new pass manager

[gmarkall]

Creating this issue to keep track of additional requests on PR #1046 and to continue the discussion - points raised, with some light editing:

• I think it might be helpful if the ModuleSimplificationPipeline was also exposed. That could be used to simplify whole modules before they are linked into multiple different modules.
• I also wonder how hard it would be to expose some of the extension points for the pipelines, so for instance the registerPipelineEarlySimplificationEPCallback method on PassBuilder? Maybe that would be a nice way of adding the refpune pass to reasonable locations inside a pipeline.

cc @aseyboldt @yashssh