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/engine.rs

@@ -147,7 +147,7 @@ where
         let mut entry_sets = IndexVec::from_elem(bottom_value.clone(), body.basic_blocks());
         analysis.initialize_start_block(body, &mut entry_sets[mir::START_BLOCK]);
 
-        if A::Direction::is_backward() && entry_sets[mir::START_BLOCK] != bottom_value {
+        if A::Direction::IS_BACKWARD && entry_sets[mir::START_BLOCK] != bottom_value {
             bug!("`initialize_start_block` is not yet supported for backward dataflow analyses");
         }
 
@@ -200,7 +200,7 @@ where
         let mut dirty_queue: WorkQueue<BasicBlock> =
             WorkQueue::with_none(body.basic_blocks().len());
 
-        if A::Direction::is_forward() {
+        if A::Direction::IS_FORWARD {
             for (bb, _) in traversal::reverse_postorder(body) {
                 dirty_queue.insert(bb);
             }