freedreno/ir3: fix failed assert in grouping

freedreno/ir3: fix failed assert in grouping

Turns out there are scenarios where we need to insert mov's in "front"
of an input.  Triggered by shaders like:

  VERT
  DCL IN[0]
  DCL IN[1]
  DCL OUT[0], POSITION
  DCL OUT[1], GENERIC[9]
  DCL SAMP[0]
  DCL TEMP[0], LOCAL
    0: MOV TEMP[0].xy, IN[1].xyyy
    1: MOV TEMP[0].w, IN[1].wwww
    2: TXF TEMP[0], TEMP[0], SAMP[0], 1D_ARRAY
    3: MOV OUT[1], TEMP[0]
    4: MOV OUT[0], IN[0]
    5: END

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