[opt-viewer] Python 3 support in opt-viewer.py

Summary:
Minor changes that allow opt-stats.py to support both Python 2 and 3.
In addition to the same dictionary iterator changes that were necessary
in https://reviews.llvm.org/D34564, this diff also:

* Explcitly converts strings to bytes when reading from and writing to stdin
  and stdout.
* No longer uses dictionaries as a sort key for optimization remarks.
  Dictionary sort order in Python 2 is pretty esoteric anyway, so it's
  not clear that the additional sorting had a benefit for end users
  (for details, https://stackoverflow.com/a/3484456/679254 is a good
  resource on Python 2 dictionary sort order).

Reviewers: anemet, davidxl

Reviewed By: anemet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34647

llvm-svn: 306720