intel/fs: Fix NULL destinations on 3-source instructions again after late DCE

intel/fs: Fix NULL destinations on 3-source instructions again after late DCE

We considered moving this down near the call to
insert_gen4_send_dependency_workarounds.  By that point it's too late
for a couple reasons.  One, we're potentially increasing resiter
pressure that may lead to anoter spill.  Two, fixup_3src_null_dest tries
to allocate a VGRF, but the post-register allocation shader uses
physical registers.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2621
Fixes: ba2fa1ceaf4 ("intel/fs: Do cmod prop again after scheduling")
Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4155>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4155>