gallivm: don't cache shaders that use fetch functions.

gallivm: don't cache shaders that use fetch functions.

This needs to be reworked, but it's a bit messy as we have to store
all the fetch pointers to be added as globals later once gallivm
has been initialised further. For now just refuse to cache shaders
that hit these paths (mainly ETC1 and BPTC).

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5049>