freedreno/ir3: fix potential segfault in RA

freedreno/ir3: fix potential segfault in RA

Triggered by shaders like:

  FRAG
  PROPERTY FS_COLOR0_WRITES_ALL_CBUFS 1
  DCL OUT[0], COLOR
  DCL CONST[0]
  DCL TEMP[0..2], LOCAL
    0: IF CONST[0].xxxx :0
    1:   MOV TEMP[0], TEMP[1]
    2: ELSE :0
    3:   MOV TEMP[0], TEMP[2]
    4: ENDIF
    5: MOV OUT[0], TEMP[0]
    6: END

not really a sane shader, although driver segfaulting is probably
not the appropriate response.

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