[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