i965/fs: Fix lowering of integer multiplication with cmod.

i965/fs: Fix lowering of integer multiplication with cmod.

If the multiplication's result is unused, except by a conditional_mod,
the destination will be null. Since the final instruction in the lowered
sequence is a partial-write, we can't put the conditional mod on it and
we have to store the full result to a register and do a MOV with a
conditional mod.

Cc: "10.6" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90580
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>