These correspond to the EmitVertex and EndPrimitive functions in GLSL.

v2 (Paul Berry <stereotype441@gmail.com>): Add stub implementations of
new pure visitor functions to i965's vec4_visitor and fs_visitor
classes.

v3 (Paul Berry <stereotype441@gmail.com>): Rename classes to be more
consistent with the names used in the GL spec.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

Currently, ir_call can be used as either a statement (for void
functions) or a value (for non-void functions).  This is rather awkward,
as it's the only class that can be used in both forms.

A number of places use ir_call::get_error_instruction() to construct a
generic value of error_type.  If ir_call is to become a statement, it
can no longer serve this purpose.

Unfortunately, none of our classes are particularly well suited for
this, and creating a new one would be rather aggrandizing.  So, this
patch introduces ir_rvalue::error_value(), a static method that creates
an instance of the base class, ir_rvalue.  This has the nice property
that you can't accidentally try and access uninitialized fields (as it
doesn't have any).  The downside is that the base class is no longer
abstract.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

This is just a subclass of ir_visitor with empty implementations of all
the visit methods for non-control flow nodes.

Used to avoid duplicating that in ir_visitor subclasses.

ir_hierarchical_visitor is another way to solve this, but is less natural
for some applications.