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

Bug b/37496338

Change-Id: I9a1dc91359daea5f4391a137b7f9e03bd941146b
Reviewed-on: https://chromium-review.googlesource.com/688057Reviewed-by: Jim Stichnoth <stichnot@chromium.org>
Reviewed-on: https://swiftshader-review.googlesource.com/12668Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>