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>

src/intel/compiler/brw_fs_builder.h

@@ -406,7 +406,7 @@ namespace brw {
          const dst_reg chan_index = vgrf(BRW_REGISTER_TYPE_UD);
          const dst_reg dst = vgrf(src.type);
 
-         ubld.emit(SHADER_OPCODE_FIND_LIVE_CHANNEL, chan_index);
+         ubld.emit(SHADER_OPCODE_FIND_LIVE_CHANNEL, chan_index)->flag_subreg = 2;
          ubld.emit(SHADER_OPCODE_BROADCAST, dst, src, component(chan_index, 0));
 
          return src_reg(component(dst, 0));