NewGVN: Clean up how we handle the INITIAL class so that everything in

it is dead or unreachable, as it should be.
This also makes the leader of INITIAL undef, enabling us to handle
irreducibility properly.

Summary:
This lets us verify, more than we do now, that we didn't screw up
value numbering.

Reviewers: davide

Subscribers: Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D29842

llvm-svn: 294844