[MSSA] Use depth first iterator instead of custom version.

[MSSA] Use depth first iterator instead of custom version.

Summary:
Originally the plan was to use the custom worklist to do some block popping,
and because we don't actually need a visited set. The custom one we have
here is slightly broken, and it's not worth fixing vs using depth_first_iterator since we aren't going to go the route we originally
were.

Fixes PR28874
Reviewers: george.burgess.iv

Subscribers: llvm-commits, gberry

Differential Revision: https://reviews.llvm.org/D23187

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277880 91177308-0d34-0410-b5e6-96231b3b80d8