radeonsi: remove VS epilog code, compile VS with PrimID export on demand

radeonsi: remove VS epilog code, compile VS with PrimID export on demand

The use of PrimID in the pixel shader is too rare to deserve such
a sizable support code.

The initial idea of the VS epilog was to move the clipping code there and
remove it based on states, but optimized variants are now used to do that
and are easier to support, so the VS epilog has turned out to be not so
useful.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>