[NewGVN] We don't use postdom info anymore. Update.
[NewGVN] We don't use postdom info anymore. Update.

Differential Revision:  https://reviews.llvm.org/D28842

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@292421 91177308-0d34-0410-b5e6-96231b3b80d8

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