When generating IR for movsd mem->reg, don't first write the entire

When generating IR for movsd mem->reg, don't first write the entire
guest reg with zeroes and then overwrite the lower half.  This forces
the back end to generate code which creates huge write-after-write
stalls in the memory system of P4s due to the different sized writes.
This apparently small change reduces the run-time of one
sse2-intensive floating point program from 145 seconds to 90 seconds
(--tool=none).

git-svn-id: svn://svn.valgrind.org/vex/trunk@1121