Fix incorrect decoding of AVX2 insns:
  VPBROADCASTB xmm, xmm
  VPBROADCASTB xmm, ymm
  VPBROADCASTW xmm, xmm
  VPBROADCASTW xmm, ymm 
Fixes #340725.



git-svn-id: svn://svn.valgrind.org/vex@2988 8f6e269a-dfd6-0310-a8e1-e2731360e62c