The PLN instruction is no more. Its functionality is now implemented
using two MAD instructions with the new native-float type. Instead of

   pln(16) r20.0<1>:F r10.4<0;1,0>:F r4.0<8;8,1>:F

we now have

   mad(8) acc0<1>:NF r10.7<0;1,0>:F r4.0<8;8,1>:F r10.4<0;1,0>:F
   mad(8) r20.0<1>:F acc0<8;8,1>:NF r5.0<8;8,1>:F r10.5<0;1,0>:F
   mad(8) acc0<1>:NF r10.7<0;1,0>:F r6.0<8;8,1>:F r10.4<0;1,0>:F
   mad(8) r21.0<1>:F acc0<8;8,1>:NF r7.0<8;8,1>:F r10.5<0;1,0>:F

... and in the case of SIMD8 only the first pair of MAD instructions is
used.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>