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

When I was preparing r218879 for commit, I removed an early return
that I decided was just noise. It wasn't. This is r218879 no-crash
edition.

This reverts commit r218881, reapplying r218879.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218887 91177308-0d34-0410-b5e6-96231b3b80d8