intel/brw: Make brw_reg::bits publicly accessible from fs_reg

I want to be able to hash an fs_reg, including all the brw_reg fields.
It's easiest to do this if I can use the "bits" union field that
incorporates many of the other ones.

We also move the using declaration for "nr" down because that field was
moved to the second section a while back.

Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29624>

src/intel/compiler/brw_ir_fs.h

@@ -75,8 +75,9 @@ public:
    using brw_reg::abs;
    using brw_reg::address_mode;
    using brw_reg::subnr;
-   using brw_reg::nr;
+   using brw_reg::bits;
 
+   using brw_reg::nr;
    using brw_reg::swizzle;
    using brw_reg::writemask;
    using brw_reg::indirect_offset;