[opt-viewer] Listify `dict_items` for Py3 indexing

Summary:
In Python 2, calling `dict.items()` returns an indexable `list`, whereas
on Python 3 it returns a set-like `dict_items` object, which cannot be
indexed. Explicitly onvert the `dict_items` object so that it can be
indexed when using Python 3.

In combination with D36622, D36623, and D36624, this change allows
`opt-viewer.py` to exit successfully when run with Python 3.4.

Test Plan:
Run `opt-viewer.py` using Python 3.4 and confirm it does not encounter a
runtime error when when indexing into `dict.items()`.

Reviewers: anemet

Reviewed By: anemet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36630

llvm-svn: 310810