Fix symmetry of Maybe() predicate. Fix bug in NowContains() predicate.

Add tests for TypeImpl::Of(), TypeImpl::NowOf() and
TypeImpl::NowContains(). Improves the implementation of
TypeImpl::NowIs() to match that of TypeImpl::NowContains().

Mark test-types with NO_VARIANTS to speedup testing, since
the variants do not affect the type system at all.

Also improve test coverage for types.

TEST=cctest/test-types
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/230673002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20639 ce2b1a6d-e550-0410-aec6-3dcde31c8c00