When SSE4.1 isn't available, we were infinitely calling PackUnsigned()
recursively.

Bug b/37496082

Change-Id: Ie1fa8150d64740776c5b80baf07d997dde9d51be
Reviewed-on: https://swiftshader-review.googlesource.com/12929Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Casey Dahlin <sadmac@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>