[MBP] Partial tail duplication into hot predecessors

Current tail duplication embedded in MBP duplicates a BB into all or none of its predecessors without too much cost analysis. So sometimes it is duplicated into cold predecessors, and in other cases it may miss the duplication into hot predecessors.

This patch improves tail duplication in 3 aspects:

  A successor can be duplicated into part of its predecessors.
  A more fine-grained benefit analysis, combined with 1, now a successor is duplicated into hot predecessors only.
  If a successor can't be duplicated into one predecessor, it doesn't impact the duplication into other predecessors.

Differential Revision: https://reviews.llvm.org/D73387