svga: Fix relative addressing translation for pixel shaders.

svga: Fix relative addressing translation for pixel shaders.

Pixel shaders do not have address registers a#, only one
loop register aL. Our only hope is to assume the address
register is in fact a loop counter and replace it with aL.

Do not translate ARL instruction for pixel shaders -- MOVA
instruction is only valid for vertex saders.

Make it more explicit relative addressing of inputs is only valid
for pixel shaders and constants for vertex shaders.