intel/compiler: Lower SSBO and shared loads/stores in NIR

intel/compiler: Lower SSBO and shared loads/stores in NIR

We have a bunch of code to do this in the back-end compiler but it's
fairly specific to typed surface messages and the way we emit them.
This breaks it out into NIR were it's easier to do things a bit more
generally.  It also means we can easily share the code between the vec4
and FS back-ends if we wish.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>