i965/vs: Fix implied_mrf_writes() for integer division pre-gen6.

i965/vs: Fix implied_mrf_writes() for integer division pre-gen6.

Previously it would assertion fail in debug builds (though the correct
value was returned in a non-debug build).  Marking it as a candidate for
stable even though it has no current consumers in the stable branches, in
case one shows up in a later backport.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=64727
NOTE: This is a candidate for stable branches.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>