draw: handle edge flags in llvm path

draw: handle edge flags in llvm path

We just ignored them altogether. While this feature is rather old-fashioned
supporting it is actually rather trivial.
This fixes the associated piglit tests (2 gl-1.0-edgeflag, 2 gl-2.0-edgeflag
and all (7) of point-vertex-id).

v2: comment fixes, and make the use of the edgeflag in clipmask consistent
with when it's actually there (should be impossible to hit a case where the
difference would actually matter but still...)

Reviewed-by: Brian Paul <brianp@vmware.com>