Fix evaluating empty loop condition to true.

Bug 27351438

Change-Id: If15f0f91931f34af6e78ad773036b91ff5d57d01
Reviewed-on: https://swiftshader-review.googlesource.com/4902
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>