compiler: Add SYSTEM_VALUE_FIRST_VERTEX and instrinsics

This VS system value will contain the value passed as <basevertex> for
indexed draw calls or the value passed as <first> for non-indexed draw
calls. It can be used to calculate the gl_VertexID as
SYSTEM_VALUE_VERTEX_ID_ZERO_BASE plus SYSTEM_VALUE_FIRST_VERTEX.

From the OpenGL 4.6 spec, 10.4 "Drawing Commands Using Vertex Arrays":

-  Page 352:
"The index of any element transferred to the GL by DrawArraysOneInstance
is referred to as its vertex ID, and may be read by a vertex shader as
gl_VertexID.  The vertex ID of the ith element transferred is first +
i."

- Page 355:
"The index of any element transferred to the GL by
DrawElementsOneInstance is referred to as its vertex ID, and may be read
by a vertex shader as gl_VertexID.  The vertex ID of the ith element
transferred is the sum of basevertex and the value stored in the
currently bound element array buffer at offset indices + i."

Currently the gl_VertexID calculation uses SYSTEM_VALUE_BASE_VERTEX but
this will have to change when the value of gl_BaseVertex is
fixed. Currently its value is broken for non-indexed draw calls because
it must be zero but we are setting it to <first>.

v2: use SYSTEM_VALUE_FIRST_VERTEX as name for the value, instead of
SYSTEM_VALUE_BASE_VERTEX_ID (Kenneth).

v3 (idr): Rebase on Rob Clark converting nir_intrinsics.h to be
generated.  Reformat commit message to 72 columns.

Reviewed-by: Neil Roberts <nroberts@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/nir_intrinsics.py

@@ -413,6 +413,7 @@ system_value("frag_coord", 4)
 system_value("front_face", 1)
 system_value("vertex_id", 1)
 system_value("vertex_id_zero_base", 1)
+system_value("first_vertex", 1)
 system_value("base_vertex", 1)
 system_value("instance_id", 1)
 system_value("base_instance", 1)