i965/fs: Mark predicated PLN instructions with dependency hints.

i965/fs: Mark predicated PLN instructions with dependency hints.

To implement the unlit_centroid_workaround, previously we emitted

   (+f0) pln(8) g20<1>F g16.4<0,1,0>F g4<8,8,1>F { align1 1Q };
   (-f0) pln(8) g20<1>F g16.4<0,1,0>F g2<8,8,1>F { align1 1Q };

where the flag register contains the channel enable bits from g0.

Since the predicates are complementary, the pair of pln instructions
write to non-overlapping components of the destination, which is the
case that the dependency control hints are designed for.

Typically setting dependency control hints on predicated instructions
isn't safe (if an instruction doesn't execute due to the predicate, it
won't update the scoreboard, leaving it in a bad state) but since we
must have at least one channel executing (i.e., +f0 is true for some
channel) by virtue of the fact that the thread is running, we can put
the +f0 pln instruction last and set the hints:

   (-f0) pln(8) g20<1>F g16.4<0,1,0>F g2<8,8,1>F { align1 NoDDClr 1Q };
   (+f0) pln(8) g20<1>F g16.4<0,1,0>F g4<8,8,1>F { align1 NoDDChk 1Q };

Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>