spirv: propagate access qualifiers through ssa & pointer

Not only variables can be flagged as NonUniformEXT but also
expressions. We're currently ignoring it in an expression such as :

   imageLoad(data[nonuniformEXT(rIndex)], 0)

The associated SPIRV :

   OpDecorate %69 NonUniformEXT
   ...
   %69 = OpLoad %61 %68

This changes propagates access qualifiers through ssa & pointers so
that when it hits a OpLoad/OpStore style instructions, qualifiers are
not forgotten.

Fixes failure the following tests :

   dEQP-VK.descriptor_indexing.*

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 8ed583fe ("spirv: Handle the NonUniformEXT decoration")
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit 0fb61dfd)