Fix MLIR pass manager initialization: hash the pass pipeline to detect when...

Fix MLIR pass manager initialization: hash the pass pipeline to detect when initialization is needed

The current logic hashes the context to detect registration changes and re-run
the pass initialization. However it wasn't checking for changes to the
pipeline, so a pass that would get added after a first run would not be
initialized during subsequent runs.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D158377