Support vector shift by constant for ARM32.

This also fixes the encoding of right shifts with unsigned elements.

Bug b/37496338

Change-Id: I9a1dc91359daea5f4391a137b7f9e03bd941146b
Reviewed-on: https://chromium-review.googlesource.com/688057
Reviewed-by: Jim Stichnoth <stichnot@chromium.org>
5 files changed