[SampleFDO][NFC] Refactor: make SampleProfileLoaderBaseImpl a template class

This patch makes SampleProfileLoaderBaseImpl a template class so it
can be used in CodeGen transformation.

Noticeable changes:
 * use one template parameter and use IRTraits to get other used
   types an type specific functions.
 * remove the temporary "inline" keywords in previous refactor
   patch.
 * change the template function findEquivalencesFor to a regular
   function. This function has a single caller with type of
   PostDominatorTree. It's simpler to use the type directly
   because MachinePostDominatorTree is not a derived type of
   template DominatorTreeBase.

Differential Revision: https://reviews.llvm.org/D96981