[MCA] Store extra information about processor resources in the ResourceManager.

Method ResourceManager::use() is responsible for updating the internal state of
used processor resources, as well as notifying resource groups that contain used
resources.

Before this patch, method 'use()' didn't know how to quickly obtain the set of
groups that contain a particular resource unit. It had to discover groups by
perform a potentially slow search (done by iterating over the set of processor
resource descriptors).

With this patch, the relationship between resource units and groups is stored in
the ResourceManager. That means, method 'use()' no longer has to search for
groups. This gives an average speedup of ~4-5% on a release build.

This patch also adds extra code comments in ResourceManager.h to better describe
the resource mask layout, and how resouce indices are computed from resource
masks.

llvm-svn: 350387