i965/fs: Pass a BAD_FILE register to the logical FB write when oMask is unused.

This will let the optimizer know that the sample mask value is unused
so its definition can be DCE'ed.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

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

@@ -3797,7 +3797,7 @@ lower_fb_write_logical_send(const fs_builder &bld, fs_inst *inst,
       length++;
    }
 
-   if (prog_data->uses_omask) {
+   if (sample_mask.file != BAD_FILE) {
       sources[length] = fs_reg(VGRF, bld.shader->alloc.allocate(1),
                                BRW_REGISTER_TYPE_UD);
 

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

@@ -394,7 +394,8 @@ fs_visitor::emit_single_fb_write(const fs_builder &bld,
 
    const fs_reg sources[] = {
       color0, color1, src0_alpha, src_depth, dst_depth, src_stencil,
-      sample_mask, brw_imm_ud(components)
+      (prog_data->uses_omask ? sample_mask : fs_reg()),
+      brw_imm_ud(components)
    };
    assert(ARRAY_SIZE(sources) - 1 == FB_WRITE_LOGICAL_SRC_COMPONENTS);
    fs_inst *write = bld.emit(FS_OPCODE_FB_WRITE_LOGICAL, fs_reg(),