Constant fold launder of null and undef

Summary:
This might be useful because clang will add
some barriers for pointer comparisons.

Reviewers: majnemer, dberlin, hfinkel, nlewycky, davide, rsmith, amharc,
kuhar

Subscribers: davide, amharc, llvm-commits

Differential Revision: https://reviews.llvm.org/D32423

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332786 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed