spirv: Allow pointers to have a deref at the base

spirv: Allow pointers to have a deref at the base

Previously, pointers fell into two categories: index/offset for UBOs,
SSBOs, etc. and var + access chain for logical pointers.  This commit
adds another logical pointer mode that's deref + access chain.

It's tempting to think that we can just replace variable-based pointers
with deref-based or at least replace the access chain with a deref
chain.  Unfortunately, there are a few sticky bits that prevent this:

 1) We can't return deref-based pointers from OpVariable because those
    opcodes may come outside of a function so there's no place to emit
    the deref instructions.

 2) We can't always use variable-based pointers because we may not
    always know the variable.  (We do now, but he upcoming function
    rework will take that option away.)

 3) We also can't replace the access chain struct with a deref.  Due to
    the re-ordering we do in order to handle loop continues, the derefs
    we would emit as part of OpAccessChain may not dominate their uses.
    We normally fix this up with nir_repair_ssa but that generates phi
    nodes which we don't want in the middle of our deref chains.

All in all, we have no real better option than to support partial access
chains while also re-emitting the deref instructions on the spot.

Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>