re-enable xl8 sharing for xmm instructions

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on March 22, 2014 11:08:43_

Currently the xl8 sharing code and the scratch reg used in
add_dstx2_shadow_write() have a conflict. We shouldn't need the scratch in
add_dstx2_shadow_write() except in rare cases of sub-dword (e.g., pextrw),
so we should be able to use reg3 there and enable sharing. Really we need
to clean up all the register reservations using drreg. In the code, look
for "XXX issue #243".

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=1487_