Revert "Revert r347596 "Support for inserting profile-directed cache prefetches""

Summary:
This reverts commit d8517b96dfbd42e6a8db33c50d1fa1e58e63fbb9.

Fix: correct  the use of DenseMap.

Reviewers: davidxl, hans, wmi

Reviewed By: wmi

Subscribers: mgorny, eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D55088

llvm-svn: 347938