[MBP] Code cleanup #3 /NFC

This is third patch to clean up the code.

Included in this patch:
1. Further unclutter trace/chain formation main routine;
2. Isolate the logic to compute global cost/conflict detection
   into its own method;
3. Heavily document the selection algorithm;
4. Added helper hook to allow PGO specific logic to be
   added in the future.
 

llvm-svn: 272582