[NFC] Refactor loop peeling code for calculating phi invariance.

Summary:
Refactor loop peeling code by moving code for calculating phi invariance
into a separate class that does the calculation.  Redescribe and rework
the algorithm in preparation for adding increased functionality.  Add
test case that does not exhibit peeling that will be subsequently supported.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: mkazantsev (Max Kazantsev)
Differential Revision: https://reviews.llvm.org/D138232