nir/spirv: Wrap access chains in a new vtn_pointer data structure

nir/spirv: Wrap access chains in a new vtn_pointer data structure

The vtn_pointer structure provides a bit better abstraction than passing
access chains around directly.  For one thing, if the pointer just
points to a variable, we don't need the access chain at all.  Also,
pointers know what their dereferenced type is so we can avoid passing
the type in a bunch of places.  Finally, pointers can, in theory, be
extended to the case where you don't actually know what variable is
being referenced.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>