Revert "InstrProf: Don't keep a large sparse list around just to zero it"

This seems to be crashing on some buildbots. Reverting to investigate.

This reverts commit r218879.
Revert "InstrProf: Update for the LLVM API change in r218879"

r218879 has been reverted for now, this needs to go to match.

This reverts commit r218880.