Handle "vmov qDest.I32 V128{0xFFFF}" so to speak, and make the case
for a zero immediate more similar.  Verify assembled output against
GNU as.  Fixes #311318.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2693 8f6e269a-dfd6-0310-a8e1-e2731360e62c