Optimization in AsSeenFromMap.

Despite all the eyes which have traveled over this code,
we all managed to miss this:

  // Note that pre and clazz are fixed at construction
  class AsSeenFromMap(pre: Type, clazz: Symbol) {
    ...
    def apply(tp: Type): Type =
      if (skipPrefixOf(pre, clazz)) tp
      else ...
  }

Additionally, the exclusion condition in asSeenFrom contained
a useless check, here:

  // !isPossiblePrefix(clazz) alone is enough
  pre.normalize.isTrivial && !isPossiblePrefix(clazz)
Merge pull request #2008 from paulp/pr/asSeenFromOpt

Optimization in AsSeenFromMap.