[Sink] Don't move landingpads

Summary: Moving landingpads into successor basic blocks makes the
verifier sad. Teach Sink that much like PHI nodes and terminator
instructions, landingpads (and cleanuppads, etc.) may not be moved
between basic blocks.

Reviewers: majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14475

llvm-svn: 253182