[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258297
91177308-0d34-0410-b5e6-
96231b3b80d8
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