Dataflow changes and associated borrowck fix.

Dataflow changes and associated borrowck fix.

Revise rustc::middle::dataflow: one must select kill-kind when calling
add_kill. The current kill-kinds are (1.) kills associated with
ends-of-scopes and (2.) kills associated with the actual action of the
expression/pattern.

Then, use this to fix borrowck analysis so that it will not treat a
break that pops through an assignment `x = { ... break; ... }` as a
kill of the "moved-out" bit for `x`.

Fix #24267.

(incorporated review feedback.)