Improve handling of unary plus.

Simple strategy: Transform unary plus into multiplication by one directly in the
parser and remove it from the Hydrogen graph later. This gives correct type
feedback without any special stub for it.

BUG=v8:2527

Review URL: https://codereview.chromium.org/13902013

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14306 ce2b1a6d-e550-0410-aec6-3dcde31c8c00