InstrProf: Use a locally tracked current count in ComputeRegionCounts

No functional change. This just makes it more obvious that the logic
in ComputeRegionCounts only depends on the counter map and local
state.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@236370 91177308-0d34-0410-b5e6-96231b3b80d8