Cranelift: add support for cold blocks.

This PR adds a flag to each block that can be set via the frontend/builder
interface that indicates that the block will not be frequently
executed. As such, the compiler backend should place the block "out of
line" in the final machine code, so that the ordinary, more frequent
execution path that excludes the block does not have to jump around it.

This is useful for adding handlers for exceptional conditions
(slow-paths, guard violations) in a way that minimizes performance cost.

Fixes #2747.

cranelift/codegen/src/machinst/blockorder.rs

@@ -378,11 +378,24 @@ impl BlockLoweringOrder {
 
         postorder.reverse();
         let mut rpo = postorder;
+
+        // Step 3: sink any cold blocks to the end of the
+        // function. Put the "deferred last" block truly at the end;
+        // this is a correctness requirement (for fallthrough
+        // returns).
+        rpo.sort_by_key(|block| {
+            block
+                .0
+                .orig_block()
+                .map(|block| f.layout.is_cold(block))
+                .unwrap_or(false)
+        });
+
         if let Some(d) = deferred_last {
             rpo.push(d);
         }
 
-        // Step 3: now that we have RPO, build the BlockIndex/BB fwd/rev maps.
+        // Step 4: now that we have RPO, build the BlockIndex/BB fwd/rev maps.
         let mut lowered_order = vec![];
         let mut lowered_succ_ranges = vec![];
         let mut lb_to_bindex = FxHashMap::default();