[FuncSpec] Minor refactoring in statistics and debug messages.

* Remove redundant variable `NbFunctionsSpecialized` as it is no longer
  used by the cost model.
* Rename statistic `NumFuncSpecialized` to `NumSpecsCreated` as a better
  description (the old name confusingly implied number of functions we have
  created clones for).
* Same for variable `SpecializedFuncs`. Renamed to `Specializations`.
* Move debug message in the destructor (avoids repetition when MaxIters > 1).

Differential Revision: https://reviews.llvm.org/D145375