intel/fs/vec4: add missing dependency in write-on-write fixed GRFs

intel/fs/vec4: add missing dependency in write-on-write fixed GRFs

If we load constant data using pull constant SENDS, and we later load
that register with some other data, we can end up in a situation where
we don't track the initial fixed register write and therefore end up
using uninitialized registers.

This tracks write-on-write of fixed GRFs like we do for normal virtual
GRFs.

v2: Fix post_alloc_reg case (Jason)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9667>