mips64: Support for Cavium MIPS Octeon Atomic and Count Instructions.

Implement Cavium MIPS specific instructions:
baddu, pop, dpop, saa, saad, laa, laad, lai, laid, lad, ladd, law, lawd,
las, lasd, lac, lacd

Fixes BZ #327223.


git-svn-id: svn://svn.valgrind.org/vex@2865 8f6e269a-dfd6-0310-a8e1-e2731360e62c
Support movi_{16b,8b}_#imm8.  Fixes #335262.  (dimitry@google.com)



git-svn-id: svn://svn.valgrind.org/vex@2866 8f6e269a-dfd6-0310-a8e1-e2731360e62c
Support the "ishst" variant of "dmb".  Fixes #335263.  (dimitry@google.com)


git-svn-id: svn://svn.valgrind.org/vex@2867 8f6e269a-dfd6-0310-a8e1-e2731360e62c