nir: lower_non_uniform_access: iterate over instructions safely

This pass moves instructions around and adds control-flow in the
middle of blocks. We need to use nir_foreach_instr_safe to ensure that
we iterate over instructions correctly anyway.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 3bd54576 ("nir: Add a lowering pass for non-uniform resource access")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit e04cf0b6)