Auto merge of rust-lang#110820 - cjgillot:faster-dcp, r=oli-obk

Optimize dataflow-const-prop place-tracking infra

Optimization opportunities found while investigating rust-lang#110719

Computing places breadth-first ensures that we create short projections before deep projections, since the former are more likely to be propagated.

The most relevant is the pre-computation of flooded places. Callgrind showed `flood_*` methods and especially `preorder_preinvoke` were especially hot. This PR attempts to pre-compute the set of `ValueIndex` that `preorder_invoke` would visit.

Using this information, we make some `PlaceIndex` inaccessible when they contain no `ValueIndex`, allowing to skip computations for those places.

cc `@jachris` as original author

compiler/rustc_mir_dataflow/src/framework/lattice.rs

@@ -75,12 +75,12 @@ pub trait MeetSemiLattice: Eq {
 
 /// A set that has a "bottom" element, which is less than or equal to any other element.
 pub trait HasBottom {
-    fn bottom() -> Self;
+    const BOTTOM: Self;
 }
 
 /// A set that has a "top" element, which is greater than or equal to any other element.
 pub trait HasTop {
-    fn top() -> Self;
+    const TOP: Self;
 }
 
 /// A `bool` is a "two-point" lattice with `true` as the top element and `false` as the bottom:
@@ -113,15 +113,11 @@ impl MeetSemiLattice for bool {
 }
 
 impl HasBottom for bool {
-    fn bottom() -> Self {
-        false
-    }
+    const BOTTOM: Self = false;
 }
 
 impl HasTop for bool {
-    fn top() -> Self {
-        true
-    }
+    const TOP: Self = true;
 }
 
 /// A tuple (or list) of lattices is itself a lattice whose least upper bound is the concatenation
@@ -274,13 +270,9 @@ impl<T: Clone + Eq> MeetSemiLattice for FlatSet<T> {
 }
 
 impl<T> HasBottom for FlatSet<T> {
-    fn bottom() -> Self {
-        Self::Bottom
-    }
+    const BOTTOM: Self = Self::Bottom;
 }
 
 impl<T> HasTop for FlatSet<T> {
-    fn top() -> Self {
-        Self::Top
-    }
+    const TOP: Self = Self::Top;
 }