InstrProf: Replace the RegionCounter class with a simpler direct approach

This removes the RegionCounter class, which is only used as a helper
in teh ComputeRegionCounts stmt visitor. This class is just an extra
layer of abstraction that makes the code harder to follow at this
point, and removing it makes the logic quite a bit more direct.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@236364 91177308-0d34-0410-b5e6-96231b3b80d8