Fix unrolling loops with return statements.

Change-Id: I8974a6bb4137d1dfad9f5dd3a92cc9306c38294c
Reviewed-on: https://swiftshader-review.googlesource.com/5184
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>