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.c

@@ -1991,6 +1991,8 @@ nir_intrinsic_from_system_value(gl_system_value val)
       return nir_intrinsic_load_base_instance;
    case SYSTEM_VALUE_VERTEX_ID_ZERO_BASE:
       return nir_intrinsic_load_vertex_id_zero_base;
+   case SYSTEM_VALUE_FIRST_VERTEX:
+      return nir_intrinsic_load_first_vertex;
    case SYSTEM_VALUE_BASE_VERTEX:
       return nir_intrinsic_load_base_vertex;
    case SYSTEM_VALUE_INVOCATION_ID:
@@ -2066,6 +2068,8 @@ nir_system_value_from_intrinsic(nir_intrinsic_op intrin)
       return SYSTEM_VALUE_BASE_INSTANCE;
    case nir_intrinsic_load_vertex_id_zero_base:
       return SYSTEM_VALUE_VERTEX_ID_ZERO_BASE;
+   case nir_intrinsic_load_first_vertex:
+      return SYSTEM_VALUE_FIRST_VERTEX;
    case nir_intrinsic_load_base_vertex:
       return SYSTEM_VALUE_BASE_VERTEX;
    case nir_intrinsic_load_invocation_id: