It corresponds to a scalar MMX type, used mainly for vector shifts.
We no longer need these shifts, and they would have been non-trivial
to emulate with SSE2.

Change-Id: I77a94ff5a62e043b991d4d6fa9ddd436d450181d
Reviewed-on: https://swiftshader-review.googlesource.com/8249Reviewed-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <capn@google.com>