ARM: skip cmpxchg failure barrier if ordering is monotonic.

The terminal barrier of a cmpxchg expansion will be either Acquire or
SequentiallyConsistent. In either case it can be skipped if the
operation has Monotonic requirements on failure.

rdar://problem/15996804

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205535 91177308-0d34-0410-b5e6-96231b3b80d8
[mips] Implement ehb, ssnop, and pause in assembler

Summary: Add negative tests for pause

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D3246

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205537 91177308-0d34-0410-b5e6-96231b3b80d8