Memoize toSimpleName

toSimpleName is called a lot from the backend, so it makes
sense to memoize it. It would be even better to communicate
with the backend using strings, because then we would not
have to enter all these simple names in the name table.

Author: odersky

compiler/src/dotty/tools/dotc/core/Names.scala

@@ -339,7 +339,12 @@ object Names {
 
     def isSimple = false
     def asSimpleName = throw new UnsupportedOperationException(s"$debugString is not a simple name")
-    def toSimpleName = termName(toString)
+
+    private var simpleName: SimpleTermName = null
+    def toSimpleName = {
+      if (simpleName == null) simpleName = termName(toString)
+      simpleName
+    }
 
     def rewrite(f: PartialFunction[Name, Name]): ThisName =
       if (f.isDefinedAt(this)) likeSpaced(f(this))