Make it possible to assert that certain instance types are compatible wrt HAllocate.

Turns clear_next_map_word_ into a flag and adds a new method
CompatibleInstanceTypes(). This is used in BuildUncheckedStringAdd()
to ASSERT that there's no difference wrt. HAllocate in using
CONS_STRING_TYPE vs CONS_ASCII_STRING_TYPE.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/110443003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18428 ce2b1a6d-e550-0410-aec6-3dcde31c8c00