radeonsi: optimize TCS epilog when invocation 0 writes tess factors

radeonsi: optimize TCS epilog when invocation 0 writes tess factors

This removes the barrier and LDS stores and loads for tess factors
when it's possible. The removal of the barrier seems more important
to me though.

In one shader, it removes 17 * 4 bytes from the shader binary.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>