spirv: Claim support for the simple memory model

It's rather surprising that we've never actually hit this before.
Aparently, Ian's SPIR-V generator currently claims the Simple when you
don't do anything complex.  We really shouldn't assert-fail on it.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 8ab9820d)