radeonsi: generate an explicit switch instruction over vertex streams

radeonsi: generate an explicit switch instruction over vertex streams

SimplifyCFG generates a switch instruction anyway when all four streams
are present, but is simultaneously not smart enough to eliminate some
redundant jumps that it generates.

The generated assembly is still a bit silly, probably because the
control flow annotation doesn't know how to handle a switch with uniform
condition.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>