[LoopPeel][NFCI] Simplify the code to calculate peel count for PGO

This reorganizes the code as a preparation for D123865:

 * Use more descriptive names for variables
 * Simplify a condition by use an already calculated value
   for `MaxPeelCount`
 * Remove a duplicate log entry
 * Report basic values for loop costs

Differential Revision: https://reviews.llvm.org/D124388