This removes uses of the "register" keyword,
which is deprecated from C++11.

Change-Id: If302563117f7d978b6f65312fc08e447905e0cde
Reviewed-on: https://swiftshader-review.googlesource.com/4513Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>