InstrProf: Use the same names for variables as we use in the profile

There's no need to use different names for the local variables than we
use in the profile itself, and it's a bit simpler and easier to debug
if we're consistent.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223173 91177308-0d34-0410-b5e6-96231b3b80d8