Fix cast bug

• A small bug was seen when 2 kernels were created on the addition of a ...gbarrier.
• But along with the 2 kernels, the temporary globals were also getting replicated leading to a compilation being thrown by OpenCL because of creation of multiple global variables sharing the same name.
• This commit attempts to avoid it. It has been tested on a Mass matrix kernel emitted by firedrake-project and has found to work.