This commit adds a hook that allows passes, analysis or data structures that can
be registered as receivers of delete notifications to decide if they want to
receive notifications All of the analysis and the currently executing passes are
automatically registered to receive notifications and this hook is useful in
reducing the runtime overhead. With this change the only analysis that accepts
notifications is alias analysis.
This commit adds the basic support for delete notification handlers. The SIL
Module is notified every time an instruction is deleted. The module will forward
notification messages to users who ask to be notified. The motivation for this
work is described in the upcoming commit to OptimizerDesign.md.
