Also fixes the corresponding emit methods for vpush and vpop to match
constraint that the maximum number of consecutive registers that can
be pushed/popped is 16.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1532233002 .