intel/fs: Allow limited copy propagation of a LOAD_PAYLOAD into another.

intel/fs: Allow limited copy propagation of a LOAD_PAYLOAD into another.

This is particularly useful in cases where register coalaesce is
unlikely to succeed because the LOAD_PAYLOAD isn't a plain copy --
E.g. when a LOAD_PAYLOAD is shuffling the contents of a barycentric
vector in order to transform it into the PLN layout.

This prevents the following shader-db regressions (including SIMD32
programs) in combination with the interpolation rework part of this
series.  On SKL:

   total instructions in shared programs: 18596672 -> 18976097 (2.04%)
   instructions in affected programs: 7937041 -> 8316466 (4.78%)
   helped: 39
   HURT: 67427

   LOST:   466
   GAINED: 220

On SNB:

   total instructions in shared programs: 13993866 -> 14202963 (1.49%)
   instructions in affected programs: 7611309 -> 7820406 (2.75%)
   helped: 624
   HURT: 52943

   LOST:   6
   GAINED: 18

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