[Verifier] Fix performance regression for LTO builds
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@258297 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm