do `move_outs` computation lazilly?

[nikomatsakis]

If I am not mistaken, the move_outs dataflow computation is only used for error reporting. However, we do the computation unconditionally here:

rust/src/librustc_mir/borrow_check/mod.rs

Lines 188 to 196 in d5a448b

	let flow_move_outs = FlowAtLocation::new(do_dataflow(
	tcx,
	mir,
	id,
	&attributes,
	&dead_unwinds,
	MovingOutStatements::new(tcx, mir, &mdpe),
	|bd, i| DebugFormatted::new(&bd.move_data().moves[i]),
	));

Perhaps it would be more efficient to wait and do it only if we find an error? Or, maybe even better, not do it at all but instead -- when we have an error -- just do a backwards DFS or something? i.e., just do it for the specific error path?

cc @pnkfelix

[nikomatsakis]

Doing a backward DFS would also be better for Polonius I think.

@spastorino is going to investigate.