intel/ir: Allow arbitrary scratch flag registers for SHADER_OPCODE_FIND_LIVE_CHANNEL.

intel/ir: Allow arbitrary scratch flag registers for SHADER_OPCODE_FIND_LIVE_CHANNEL.

This shouldn't cause any functional change at this point, it changes
SHADER_OPCODE_FIND_LIVE_CHANNEL to use the flag register specified at
the IR level instead of the hard-coded f1.0, now that it can be
represented in backend_instruction::flag_subreg.  This will be
necessary for scheduling to behave correctly once more things start
making use of f1.0.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>