radeonsi: set is_monolithic for VS prologs when the shader is really monolithic

radeonsi: set is_monolithic for VS prologs when the shader is really monolithic

This fixes a bug with NGG that is probably harmless.

Basically, !is_monolithic makes the VS prolog emit
llvm.amdgcn.init.exec.from.input, which sets the EXEC mask to only enable
ES threads. In the NGG non-GS case, the GS threads <= ES threads, so it was
never an issue.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3095>