Auto merge of #52612 - matthewjasper:remove-unnecessary-flow, r=nikomatsakis

Don't keep the possibly initialized flow around longer than needed

The possibly initialized flow isn't used after liveness is computed, so don't keep it around. Locally this is about a 10% time win for tuple-stress (which is spending a lot of time calculating flows now that it's not spending so much on liveness).

r? @nikomatsakis

diff --cc src/librustc_mir/borrow_check/mod.rs
Simple merge