StorageLive of live local when building tokio with -Zcross-crate-inline-threshold=always + high MIR inlining threshold

[saethlin]

Reproduces with https://github.com/tokio-rs/tokio on commit 19d96c067410e30da29978be3513824645902a84

cd tokio/tokio
RUSTFLAGS="-Ztreat-err-as-bug -Zcross-crate-inline-threshold=always -Zinline-mir-hint-threshold=10000 -Zinline-mir-threshold=10000" cargo b --release --features sync

error: internal compiler error: broken MIR in Item(DefId(0:1070 ~ tokio[56a2]::sync::broadcast::{impl#14}::blocking_recv)) (after phase change to runtime-optimized) at bb140[16]:
                                StorageLive(_326) which already has storage here
    --> tokio/src/sync/broadcast.rs:1255:13
     |
1255 |         fut.await
     |             ^^^^^

Remove -Ztreat-err-as-bug and add -Zvalidate-mir to get a lot of ICEs like this:
rustc-ice-2023-11-09T01_05_17-2728619.txt

[RalfJung]

So, looks like we should either allow StorageLive on an already live local, or we need to ensure that all locals are explicitly marked StorageDead before returning. No idea how realistic it is to achieve the latter.

Cc @cjgillot @matthewjasper