[llvm-mca] Let the Scheduler notify dispatch stall events caused by the lack...

[llvm-mca] Let the Scheduler notify dispatch stall events caused by the lack of scheduling resources.

This patch moves part of the logic that notifies dispatch stall events from the
DispatchUnit to the Scheduler.

The main goal of this patch is to remove (yet another) dependency between the
DispatchUnit and the Scheduler. Before this patch, the DispatchUnit had to know
about `Scheduler::Event` and how to classify stalls due to the lack of scheduling
resources. This patch removes that knowledge and simplifies the logic in
DispatchUnit::checkScheduler.

This is another change done in preparation for the work to fix PR36663.

No functional change intended.

llvm-svn: 329835