x86-64, NUMA: Remove %NULL @nodeids handling from compute_hash_shift()

numa_emulation() called compute_hash_shift() with %NULL @nodeids which
meant identity mapping between index and nodeid.  Make
numa_emulation() build identity array and drop %NULL @nodeids handling
from populate_memnodemap() and thus from compute_hash_shift().  This
is to prepare for transition to using memblks instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Shaohui Zheng <shaohui.zheng@intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: H. Peter Anvin <hpa@linux.intel.com>