Add support to the JSCallReducer to recognize JSConstruct nodes where
the target is the Object constructor, and reduce them to JSCreate
nodes if either

 (a) no value is passed to the Object constructor, or
 (b) the target and new.target are definitely not identical, by checking
     whether both target and new.target are different HeapConstants
     (if they are not, then the JSCreateLowering will not be able to
     do a lot with the JSCreate anyways).

This should cover the relevant cases for subclassing appropriately. It
fixes the 3-4x slowdown on the micro-benchmark mentioned in the linked
bug,

  baseNoExtends: 752 ms.
  baseExtendsObject: 752 ms.
  baseExtendsViaFactory: 751 ms.

and thus removes the performance cliff.

R=jarin@chromium.org

Bug: v8:6801
Change-Id: Id265fd1399302a67b5790a6d0156679920c58bdd
Reviewed-on: https://chromium-review.googlesource.com/657019Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47913}