[instcombine] PRE freeze to only potentially posion/undef operand of phi

This extends the foldOpIntoPhi code used when visiting a freeze user of a phi to allow any non-undef/poison operand as opposed to only non-undef/poison constants.  This lets us hoist a freeze in the increment of an IV into the preheader in many cases.

Differential Revision: https://reviews.llvm.org/D111744