glsl: drop cache_fallback

This turned out to be a dead end, it is much easier and less error
prone to just cache the IR used by the drivers backend e.g. TGSI or
NIR.

Cc: "17.2 17.3" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit cf05bb50)