x86 front ends: tighten up decoding of MOV Ib,Eb and MOV Iv,Ev.  This
failed to check the g-register in the modrm byte, with the result that
it will mis-decode the AVX2 XABORT and XBEGIN instructions as these
instead, with obviously-bizarre consequences.



git-svn-id: svn://svn.valgrind.org/vex/trunk@2768 8f6e269a-dfd6-0310-a8e1-e2731360e62c