Clean up manual bit field usage in PreParserExpression

Instead of using an integer value and manual bit-fiddling, use C++'s
support for specifying bit sizes for integral types. This way the bits
used to describe a PreParserExpression are handled by the compiler.

BUG=
LOG=
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/422363002

Patch from Adrian Perez <aperez@igalia.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24429 ce2b1a6d-e550-0410-aec6-3dcde31c8c00