[PM] Fix a bug where we didn't properly clear the list map when the list

[PM] Fix a bug where we didn't properly clear the list map when the list
became empty. This would manifest later as an assert failure due to
a non-empty list map but an empty result map. This doesn't easily
manifest with just the module pass manager and the function pass
manager, but the next commit will add the CGSCC pass manager that hits
this assert immediately.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206744 91177308-0d34-0410-b5e6-96231b3b80d8