Add a couple more constant folding rules for vectors.


git-svn-id: svn://svn.valgrind.org/vex@2844 8f6e269a-dfd6-0310-a8e1-e2731360e62c
Implement TBL and TBX instructions.


git-svn-id: svn://svn.valgrind.org/vex@2845 8f6e269a-dfd6-0310-a8e1-e2731360e62c