Support extra instruction bits and pieces, enough to get Firefox started:
* more scalar int <-> FP conversions
* more vector integer narrowing
* a few more vector shift by imm cases
* FCVTAS (kludged)


git-svn-id: svn://svn.valgrind.org/vex/trunk@2833 8f6e269a-dfd6-0310-a8e1-e2731360e62c