intel/compiler: Unset flag reg when FB write is not predicated

In the FS IR we pretend that the instruction is predicated with (+f0.1)
just for flag dependency tracking purposes. Since the instruction
doesn't support predication before Haswell, we unset the predicate so we
should also unset the flag register so that we can round-trip the
disassembly.

Reviewed-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>