Chrome and many other products require SSE2 support as a minimum. Note
that MMX checks are left in place for now. Dead code paths are removed.

Bug swiftshader:78

Change-Id: Iabd8b1dc2092949d5dba29a78e75d014e808f12c
Reviewed-on: https://swiftshader-review.googlesource.com/11068Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>