gallivm: Compile flag to debug TGSI execution through printfs.

gallivm: Compile flag to debug TGSI execution through printfs.

It is similar to tgsi_exec.c's DEBUG_EXECUTION compile flag.

I had prototyped this for a while while debugging an issue, but finally
cleaned this up and added a few more bells and whistles.

v2: Use '$' as marker; better output. Thanks to Brian, Zack and Roland
reviews.

Here is a sample output.

    CONST[0].x =  0.00625000009 0.00625000009 0.00625000009 0.00625000009
    CONST[0].y =  -0.00714285718 -0.00714285718 -0.00714285718 -0.00714285718
    CONST[0].z =  -1 -1 -1 -1
    CONST[0].w =  1 1 1 1
    IN[0].x =  143.5 175.5 175.5 143.5
    IN[0].y =  123.5 123.5 155.5 155.5
    IN[0].z =  0 0 0 0
    IN[0].w =  1 1 1 1
$   1: RCP TEMP[0].w, IN[0].wwww
    TEMP[0].w =  1 1 1 1
$   2: MAD TEMP[0].xy, IN[0], CONST[0], CONST[0].zwzw
    TEMP[0].x =  -0.103124976 0.0968750715 0.0968750715 -0.103124976
    TEMP[0].y =  0.117857158 0.117857158 -0.110714316 -0.110714316
$   3: MUL OUT[0].xy, TEMP[0], TEMP[0].wwww
    OUT[0].x =  -0.103124976 0.0968750715 0.0968750715 -0.103124976
    OUT[0].y =  0.117857158 0.117857158 -0.110714316 -0.110714316
$   4: MUL OUT[0].z, IN[0].zzzz, TEMP[0].wwww
    OUT[0].z =  0 0 0 0
$   5: MOV OUT[0].w, TEMP[0]
    OUT[0].w =  1 1 1 1
$   6: END
    OUT[0].x =  -0.103124976 0.0968750715 0.0968750715 -0.103124976
    OUT[0].y =  0.117857158 0.117857158 -0.110714316 -0.110714316
    OUT[0].z =  0 0 0 0
    OUT[0].w =  1 1 1 1