Two minor cleanups in pattern rewriter

Two minor cleanups in pattern rewriter

- No need to call AsVariableProxy() on a VariableProxy
- Reduce AST visitor boilerplate using pre-existing macro
  (and re-alphabetize the list of non-patterns).

R=littledan@chromium.org

Review URL: https://codereview.chromium.org/1376633005

Cr-Commit-Position: refs/heads/master@{#31032}