Move always_storage_live_locals.

It's very closely related to `MaybeStorageLive` and `MaybeStorageDead`.
It's weird that it's currently in a different module.

compiler/rustc_mir_dataflow/src/impls/mod.rs

@@ -15,4 +15,6 @@ pub use self::initialized::{
 pub use self::liveness::{
     MaybeLiveLocals, MaybeTransitiveLiveLocals, TransferFunction as LivenessTransferFunction,
 };
-pub use self::storage_liveness::{MaybeRequiresStorage, MaybeStorageDead, MaybeStorageLive};
+pub use self::storage_liveness::{
+    MaybeRequiresStorage, MaybeStorageDead, MaybeStorageLive, always_storage_live_locals,
+};

compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs

@@ -7,6 +7,23 @@ use rustc_middle::mir::*;
 use super::MaybeBorrowedLocals;
 use crate::{Analysis, GenKill, ResultsCursor};
 
+/// The set of locals in a MIR body that do not have `StorageLive`/`StorageDead` annotations.
+///
+/// These locals have fixed storage for the duration of the body.
+pub fn always_storage_live_locals(body: &Body<'_>) -> BitSet<Local> {
+    let mut always_live_locals = BitSet::new_filled(body.local_decls.len());
+
+    for block in &*body.basic_blocks {
+        for statement in &block.statements {
+            if let StatementKind::StorageLive(l) | StatementKind::StorageDead(l) = statement.kind {
+                always_live_locals.remove(l);
+            }
+        }
+    }
+
+    always_live_locals
+}
+
 pub struct MaybeStorageLive<'a> {
     always_live_locals: Cow<'a, BitSet<Local>>,
 }