nir: Take call instruction into account in copy_prop_vars

nir: Take call instruction into account in copy_prop_vars

Calls are not used yet (functions are inlined), but since new code is
already taking them into account, do it here too.  The convention here
and in other places is that no writable memory is assumed to remain
unchanged, as well as global variables.

Also, explicitly state the modes affected (instead of using the
reverse logic) in one of the apply_for_barrier_modes calls.

Suggested by Jason.

v2: Consider local vars used by a call to be conservative, SPIR-V has
    such cases. (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>