Since the empty string is canonical HeapObject now, we can use
this fact to optimize

  - strict equality comparisons with the empty string to a
    simple ReferenceEqual operation, and
  - optimize ToBoolean to avoid instance type checks completely.

Drive-by-fix: Allow InternalizedString for Type::HeapConstant
in the type system. This is safe, since InternalizedStrings
can be compared to other heap constants by reference (except
for non-InternalizedStrings, which are excluded from the
HeapConstant type).

BUG=v8:5267
R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2681273002
Cr-Commit-Position: refs/heads/master@{#43050}