Mark Knights Landing as having slow two memory operand instructions

Summary: Knights Landing, because it is Atom derived, has slow two memory operand instructions. Mark the Knights Landing CPU model accordingly.

Patch by David Zarzycki.

Reviewers: craig.topper

Reviewed By: craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37224

llvm-svn: 311979