Change Direction::{is_forward,is_backward} functions into constants

Make it explicit that the analysis direction is constant.

This also makes the value immediately available for optimizations.
Previously those functions were neither inline nor generic and so their
definition was unavailable when using data flow framework from other
crates.

compiler/rustc_mir_dataflow/src/framework/direction.rs

@@ -9,11 +9,9 @@ use super::{
 };
 
 pub trait Direction {
-    fn is_forward() -> bool;
+    const IS_FORWARD: bool;
 
-    fn is_backward() -> bool {
-        !Self::is_forward()
-    }
+    const IS_BACKWARD: bool = !Self::IS_FORWARD;
 
     /// Applies all effects between the given `EffectIndex`s.
     ///
@@ -68,9 +66,7 @@ pub trait Direction {
 pub struct Backward;
 
 impl Direction for Backward {
-    fn is_forward() -> bool {
-        false
-    }
+    const IS_FORWARD: bool = false;
 
     fn apply_effects_in_block<'tcx, A>(
         analysis: &A,
@@ -338,9 +334,7 @@ where
 pub struct Forward;
 
 impl Direction for Forward {
-    fn is_forward() -> bool {
-        true
-    }
+    const IS_FORWARD: bool = true;
 
     fn apply_effects_in_block<'tcx, A>(
         analysis: &A,