spirv: Use offset_pointer_dereference to instead of get_vulkan_resource_index

spirv: Use offset_pointer_dereference to instead of get_vulkan_resource_index

There is no good reason why we should have the same logic repeated in
get_vulkan_resource_index and vtn_ssa_offset_pointer_dereference.  If
we're a bit more careful about how we do things, we can just use the one
function and get rid of the other entirely.  This also makes the push
constant special case a lot more clear.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>