gl_InstanceID implementation

I need some help connecting the dots here.
I can see a few ways of doing it, but it
isn't trivial for me to see which one
would be right. Could you tell me how the
value should go from the VertexProcessor to
being assigned a value in the actual shader.

Change-Id: Ie0bc2024f51a904ab8144c435b623cfefe8e6704
Reviewed-on: https://swiftshader-review.googlesource.com/2920
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>

src/OpenGL/compiler/ParseHelper.cpp

diff --git a/src/OpenGL/compiler/ParseHelper.cpp b/src/OpenGL/compiler/ParseHelper.cpp
index 2d9b3d5..4fda827 100644
--- a/src/OpenGL/compiler/ParseHelper.cpp
+++ b/src/OpenGL/compiler/ParseHelper.cpp
@@ -323,6 +323,7 @@
     case EvqFragCoord:      message = "can't modify gl_FragCoord";   break;
     case EvqFrontFacing:    message = "can't modify gl_FrontFacing"; break;
     case EvqPointCoord:     message = "can't modify gl_PointCoord";  break;
+    case EvqInstanceID:     message = "can't modify gl_InstanceID";  break;
     default:
 
         //