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

llvm-svn: 332786