Subzero: Remove a TODO comment about shld/shrd.

The 32-bit validator is now consistent with the 64-bit
validator w.r.t. 16-bit shld/shrd and accepts it. We didn't
really use the 16-bit form in Subzero though, only the 32-bit
one for 64-bit ops, I think.

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/696753003