radeonsi: fix VertexID for OpenGL

This fixes all failing piglit VertexID tests.

Cc: 10.4 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
diff --git a/src/gallium/drivers/radeonsi/si_shader.c b/src/gallium/drivers/radeonsi/si_shader.c
index 1694490..e93708b 100644
--- a/src/gallium/drivers/radeonsi/si_shader.c
+++ b/src/gallium/drivers/radeonsi/si_shader.c
@@ -591,8 +591,11 @@
 		break;
 
 	case TGSI_SEMANTIC_VERTEXID:
-		value = LLVMGetParam(radeon_bld->main_fn,
-				     si_shader_ctx->param_vertex_id);
+		value = LLVMBuildAdd(gallivm->builder,
+				     LLVMGetParam(radeon_bld->main_fn,
+						  si_shader_ctx->param_vertex_id),
+				     LLVMGetParam(radeon_bld->main_fn,
+						  SI_PARAM_BASE_VERTEX), "");
 		break;
 
 	case TGSI_SEMANTIC_SAMPLEID: