Fix #6341: Revise purity checking

There is some confusion on terms that needs to be fixed.

Step 1: Make `SimplyPure apply to paths only.

The only point where it is used is in eta expansion to answer the question
whether in a partial application `f(e, _)` the expression `e` should be
lifted out, giving `{ val x = e; y => f(x, y) }` instead of `y => f(e, y)`.
With the change, closures and new expressions are never simply pure, so
are always lifted out. This can reduce the number of allocations, if the
lambda is applied several times.

Author: odersky

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -367,7 +367,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
   }
 
   /** The purity level of this expression.
-   *  @return   SimplyPure  if expression has no side effects and cannot contain local definitions
+   *  @return   SimplyPure  if expression has no side effects is a path
    *            Pure        if expression has no side effects
    *            Idempotent  if running the expression a second time has no side effects
    *            Impure      otherwise
@@ -381,16 +381,15 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
     case EmptyTree
        | This(_)
        | Super(_, _)
-       | Literal(_)
-       | Closure(_, _, _) =>
+       | Literal(_) =>
       SimplyPure
     case Ident(_) =>
       refPurity(tree)
     case Select(qual, _) =>
       if (tree.symbol.is(Erased)) Pure
       else refPurity(tree).min(exprPurity(qual))
-    case New(_) =>
-      SimplyPure
+    case New(_) | Closure(_, _, _) =>
+      Pure
     case TypeApply(fn, _) =>
       if (fn.symbol.is(Erased)) Pure else exprPurity(fn)
     case Apply(fn, args) =>