R600: Remove softPromoteHalfType
Also includes a kind of hacky, minimal change to avoid assertions
when softPromoteHalfType is removed to fix kernel arguments
lowered as f16. Half support was never really implemented
for r600, and there just happened to be a few incidental tests
which included a half argument (which were also not even meaningful,
since the function body just folded to nothing due to no callable
function support).
