gallivm: add indirect texture switch statement builder.

gallivm: add indirect texture switch statement builder.

This adds the apis to add an indirect accessor for arrays
of textures, using an LLVM switch statement and per-texture
sampler functions.

It also adds the indexer to the sampler parameters

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3778>