Rebase to the upstream

[aqjune]

Hello bryant,

I found that the patch from LLVM Phabricator (https://reviews.llvm.org/D29865 , D29866) can be successfully rebased on the most recent LLVM commit wit minor fix.

Here is my repo: https://github.com/aqjune/llvm-memoryssa/commits/pdse

Are you working on the patches still? I'm interested in implementing PDSE, and I can be of help in updating the patches if you're not working on them anymore.

Sorry for leaving an issue at seemingly private repo, but I couldn't find your email address from anywhere :(