[MCA] Always check if scheduler resources are unavailable when reporting dispatch...

[MCA] Always check if scheduler resources are unavailable when reporting dispatch stalls.

Dispatch stall cycles may be associated to multiple dispatch stall events.
Before this patch, each stall cycle was associated with a single stall event.
This patch also improves a couple of code comments, and adds a helper method to
query the Scheduler for dispatch stalls.

llvm-svn: 354877