[opt-viewer] Use Python 3-compatible iteritems

Summary:
Replace a usage of a Python 2-specific `dict.iteritems()` with the
Python 3-compatible definition provided at the top of the same file.

Test Plan:
Run `opt-viewer.py` using Python 3 and confirm it no longer encounters a
runtime error when calling `dict.iteritems()`.

Reviewers: anemet

Reviewed By: anemet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36623

llvm-svn: 310740