Expand loop peeling phi computation to handle binary ops and casts

Summary:
Expand the capabilities of the code for computing how many peels are
needed to make phis determined.  A cast gets the peel count for the
value being casted while a binary op gets the maximum of the operands.

Respond to review comments: remove redundant asserts.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By:mkazantsev (Max Kazantsev),syzaara (Zaara Syeda)
Differential Revision: https://reviews.llvm.org/D138719