Fix checks to bit flags of PreParserExpression

This fixes checks on the "code_" member of PreParserExpression, in order
to make methods IsThis(), IsThisProperty(), IsProperty(), IsCall() and
IsValidReferenceExpression() work correctly.

BUG=v8:3456
LOG=
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/410873004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22562 ce2b1a6d-e550-0410-aec6-3dcde31c8c00