- Introduce the new union type UnionType<T1,T2>, used for Number and 
  Numeric. Similarly, PairType<T1,T2> is used for a Turbofan operation
  with two results. Further details in the design doc:
  https://docs.google.com/document/d/10foP3m7SDWyFfbda96iEJ_XYOLQdd32AeoP8SD1cTcs/edit#heading=h.bghdno28mr7u
- Allow to derive the MachineType from a static type. 
  This allows to select the right MachineType when performing a 
  load/store.
- Disallow casts (UncheckedCast() or CAST()) when the target and 
  origin type have no overlap.
  New cast ReinterpretCast() is an UncheckedCast without this check.
- Caveat of this CL: Checked casts (CAST()) are not possible for 
  UnionType<T1,T2> with the exception of Number (due to the existence
  of Number in OBJECT_TYPE_LIST and the existence of an IsNumber() 
  function in the runtime).

Bug: v8:6949
Change-Id: I21a683d1341f69cebd8a347f545b454b463c52ad
Reviewed-on: https://chromium-review.googlesource.com/723320
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48796}