STRD (both ARM and Thumb): for push-like cases -- specifically, STRD
rD1,rD2, [sp, #-8], generate IR for the SP writeback before the
stores.  This loses restartability of the instruction but avoids
Memcheck complaining that we're writing below the stack pointer.



git-svn-id: svn://svn.valgrind.org/vex/trunk@2714 8f6e269a-dfd6-0310-a8e1-e2731360e62c