Refactoring only: Make the handling of PropertyType more explicit.

Refactoring only: Make the handling of PropertyType more explicit.

Do not rely on 'default' clauses or 'if's when analysing a PropertyType, because
this makes it hard to find the relevant places when a new type is added. Note
that the detection of "phantom property types" is left untouched, because this
might have a performance impact, especially for the GC (to be investigated).

This is a preliminary step for introducing a new kind of map transition.

Review URL: http://codereview.chromium.org/8491016

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9900 ce2b1a6d-e550-0410-aec6-3dcde31c8c00