freedreno/ir3: consider instruction neighbors in cp

freedreno/ir3: consider instruction neighbors in cp

Fanin (merge) nodes require it's srcs to be "adjacent" in consecutive
scalar registers.  Keep track of instruction neighbors in copy-
propagation step and avoid eliminating mov's which would cause an
instruction to need multiple distinct left and/or right neighbors.

This lets us not fall on our face when we encounter things like:

  1: MOV TEMP[2], IN[0].xyzw
  2: TEX OUT[0].xy, TEMP[2], SAMP[0], SHADOW2D
  3: MOV TEMP[2].xy, IN[0].yxzz
  4: TEX OUT[0].zw, TEMP[2], SAMP[0], SHADOW2D
  5: END

Signed-off-by: Rob Clark <robclark@freedesktop.org>