[GVN,NewGVN] Keep nonnull if K does not move.

In combineMetadata, we should be able to preserve K's nonnull metadata,
if K does not move. This condition should hold for all replacements by
NewGVN/GVN, but I added a bunch of assertions to verify that.

Fixes PR35038.

There probably are additional kinds of metadata that could be preserved
using similar reasoning. This is follow-up work.

Reviewers: dberlin, davide, efriedma, nlopes

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D47339

llvm-svn: 339149