intel/fs: Use helper for discard sample mask flag subregister number.

Use it instead of hard-coding f0.1 for the sample mask of programs
that use discard.  This will make the task easier when we replace f0.1
with another flag register location in order to support discard with
SIMD32 shaders.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/intel/compiler/brw_fs.cpp

@@ -1532,7 +1532,7 @@ fs_visitor::emit_discard_jump()
     * shader if all relevant channels have been discarded.
     */
    fs_inst *discard_jump = bld.emit(FS_OPCODE_DISCARD_JUMP);
-   discard_jump->flag_subreg = 1;
+   discard_jump->flag_subreg = sample_mask_flag_subreg(this);
 
    discard_jump->predicate = BRW_PREDICATE_ALIGN1_ANY4H;
    discard_jump->predicate_inverse = true;
@@ -4286,7 +4286,7 @@ sample_mask_reg(const fs_builder &bld)
       return brw_imm_ud(0xffffffff);
    } else if (brw_wm_prog_data(v->stage_prog_data)->uses_kill) {
       assert(bld.group() < 16 && bld.dispatch_width() <= 16);
-      return brw_flag_reg(0, 1);
+      return brw_flag_subreg(sample_mask_flag_subreg(v));
    } else {
       assert(v->devinfo->gen >= 6 && bld.dispatch_width() <= 16);
       return retype(brw_vec1_grf((bld.group() >= 16 ? 2 : 1), 7),