intel/compiler: Reset default flag register in brw_find_live_channel()

intel/compiler: Reset default flag register in brw_find_live_channel()

emit_uniformize() emits SHADER_OPCODE_FIND_LIVE_CHANNEL with its
flag_subreg set, so that the IR knows which flag is accessed. However
the flag is only used on Gen7 in Align1 mode.

To avoid setting unnecessary bits in the instruction words, get the
information we need and reset the default flag register. This allows
round-tripping through the assembler/disassembler.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>