Avoid expensive computations in Stats.record calls

When `Stats.enabled` is false, `Stats.record` should be free, but before
this commit this wasn't the case: since the `n` passed to `record` was
by-value, it had to be evaluated, this is very expensive for calls like:

    record("retained typed trees after typer", unit.tpdTree.treeSize)

Before this commit, we got after inlining:

    val n = unit.tpdTree.treeSize

Now we get:

    def n = unit.tpdTree.treeSize

it'd be even nicer if we didn't generate anything at all, but this
shouldn't have much impact on running time.

Author: smarter

compiler/src/dotty/tools/dotc/util/Stats.scala

@@ -21,7 +21,7 @@ import collection.mutable
   }
 
   @inline
-  def record(fn: String, n: Int = 1) =
+  def record(fn: => String, n: => Int = 1) =
     if (enabled) doRecord(fn, n)
 
   private def doRecord(fn: String, n: Int) =