Fix guest_amd64_toIR xbegin and xtest to match cpuid given for AVX hwcaps.

Otherwise valgrind none/tests/amd64/tm1.vgtest might fail on amd64-avx.

git-svn-id: svn://svn.valgrind.org/vex/trunk@2782 8f6e269a-dfd6-0310-a8e1-e2731360e62c