[mips][microMIPS] Implement CACHE, PREF, SSNOP, EHB and PAUSE instructions

Differential Revision: http://reviews.llvm.org/D5204


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224785 91177308-0d34-0410-b5e6-96231b3b80d8