Filter compile flags based on compile options.

[erick-xanadu]

Instead of having two pipelines, (one for async and one for non-async), we could have one single pipeline by predicating the compiler passes. For example:

A_PIPELINE = (
  "A_PIPELINE_NAME",
  [
     ("a-pass", lambda options: return options.enable_a_pass), # conditionally added
     ("b-pass", lambda options: return True), # always added
  ]

And then we can just use a filter to get the final pass list.