i965: Fixup for don't dead-code eliminate instructions that write to the accumulator.

Accidentally pushed an old version of the patch.

v2: Set destination register using brw_null_reg().
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -1854,8 +1854,7 @@ fs_visitor::dead_code_eliminate()
             case BRW_OPCODE_ADDC:
             case BRW_OPCODE_SUBB:
             case BRW_OPCODE_MACH:
-               inst->dst.file = ARF;
-               inst->dst.reg = BRW_ARF_NULL;
+               inst->dst = fs_reg(retype(brw_null_reg(), inst->dst.type));
                break;
             default:
                inst->remove();

src/mesa/drivers/dri/i965/brw_vec4.cpp

@@ -322,8 +322,7 @@ vec4_visitor::dead_code_eliminate()
             case BRW_OPCODE_ADDC:
             case BRW_OPCODE_SUBB:
             case BRW_OPCODE_MACH:
-               inst->dst.file = ARF;
-               inst->dst.reg = BRW_ARF_NULL;
+               inst->dst = dst_reg(retype(brw_null_reg(), inst->dst.type));
                break;
             default:
                inst->remove();