Avoid MIR bloat in inlining

In 126578 we ended up with more binary size increases than expected.

This change attempts to avoid inlining large things into small things, to avoid that kind of increase, in cases when top-down inlining will still be able to do that inlining later.

compiler/rustc_mir_transform/src/cost_checker.rs

@@ -31,6 +31,37 @@ impl<'b, 'tcx> CostChecker<'b, 'tcx> {
         CostChecker { tcx, param_env, callee_body, instance, penalty: 0, bonus: 0 }
     }
 
+    /// Add function-level costs not well-represented by the block-level costs.
+    ///
+    /// Needed because the `CostChecker` is used sometimes for just blocks,
+    /// and even the full `Inline` doesn't call `visit_body`, so there's nowhere
+    /// to put this logic in the visitor.
+    pub fn add_function_level_costs(&mut self) {
+        fn is_call_like(bbd: &BasicBlockData<'_>) -> bool {
+            use TerminatorKind::*;
+            match bbd.terminator().kind {
+                Call { .. } | Drop { .. } | Assert { .. } | InlineAsm { .. } => true,
+
+                Goto { .. }
+                | SwitchInt { .. }
+                | UnwindResume
+                | UnwindTerminate(_)
+                | Return
+                | Unreachable => false,
+
+                Yield { .. } | CoroutineDrop | FalseEdge { .. } | FalseUnwind { .. } => {
+                    unreachable!()
+                }
+            }
+        }
+
+        // If the only has one Call (or similar), inlining isn't increasing the total
+        // number of calls, so give extra encouragement to inlining that.
+        if self.callee_body.basic_blocks.iter().filter(|bbd| is_call_like(bbd)).count() == 1 {
+            self.bonus += CALL_PENALTY;
+        }
+    }
+
     pub fn cost(&self) -> usize {
         usize::saturating_sub(self.penalty, self.bonus)
     }