[Verifier] Fix performance regression for LTO builds

Summary:
Fix a significant performance regression by introducing GlobalValueVisited field and reusing the map.
This is a follow up to r257823 that slowed down linking Chrome with LTO by 2.5x.

If you revert this commit, please, also revert r257823.

BUG=https://llvm.org/bugs/show_bug.cgi?id=26214

Reviewers: pcc, loladiro, joker.eph

Subscribers: krasin1, joker.eph, loladiro, pcc

Differential Revision: http://reviews.llvm.org/D16338

llvm-svn: 258297