tgsi: lowering support for alpha test

This emulates alpha-test with a compare + KILL_IF.  The alpha-ref value
is passed to the shader via constant tagged with new ALPHAREF semantic.
For example:

  FRAG
  PROPERTY FS_COLOR0_WRITES_ALL_CBUFS 1
  DCL IN[0], COLOR, COLOR
  DCL OUT[0], COLOR
    0: MOV OUT[0], IN[0]
    1: END

with alpha-func PIPE_FUNC_LESS, becomes:

  FRAG
  PROPERTY FS_COLOR0_WRITES_ALL_CBUFS 1
  DCL IN[0], COLOR, COLOR
  DCL OUT[0], COLOR
  IMM[0] FLT32 {    0.0000,     1.0000,   128.0000,     0.0000}
  DCL TEMP[0]
  DCL TEMP[1]
  DCL CONST[0], ALPHAREF
    0: MOV TEMP[1], IN[0]
    1: SGE TEMP[0].x, TEMP[1].wwww, CONST[0].xxxx
    2: IF TEMP[0].x :0
    3:   KILL
    4: ENDIF
    5: MOV OUT[0], TEMP[1]
    6: END

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