Fixes#171890
If the pointer operands of an instruction are all constants with generic
AS, we always infer the AS of the instruction as uninitialized finally.
And the rewrite process will skip cloning the instruction, producing
invalid IR.
This patch fixes it by inferring the AS of this kind of instruction as
flat. Maybe we can fold the operator with all constants to get better
performance, but I think this case is rare in the real world.
8b5e95b1fd