PHI nodes not in the loop header aren't part of the loop iteration initial

PHI nodes not in the loop header aren't part of the loop iteration initial
state. Furthermore, they might not have two operands. This fixes the underlying
issue behind the crashes introduced in r142781.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142788 91177308-0d34-0410-b5e6-96231b3b80d8