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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218881 91177308-0d34-0410-b5e6-96231b3b80d8