NewGVN: Make sure we don't incorrectly use PredicateInfo when doing PHI of ops

Summary: When we backtranslate expressions, we can't use the predicateinfo, since we are evaluating them in a different context.

Reviewers: davide, mcrosier

Subscribers: sanjoy, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D37174

llvm-svn: 312352