[MemorySSA] Limit clobber walks.

Summary: This patch limits all getClobberingMemoryAccess() walks to MaxCheckLimit.

Reviewers: george.burgess.iv

Subscribers: sanjoy, jlebar, Prazek, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59569

llvm-svn: 357319