This reverts commit f20261bf.

Reason for revert: We need the GetMapConstructor instruction for CallApiCallbackStub

Original change's description:
> Remove ClassOf intrinsic from FCG and CS
> 
> I plan to change the constructor field of maps, and instead of patching
> the intrinsics all over the place, just fall back to the runtime.
> 
> R=​bmeurer@chromium.org
> BUG=v8:6084
> 
> Change-Id: Ie294b74ab615fd794d7fc47488e2e30e2b49b4db
> Reviewed-on: https://chromium-review.googlesource.com/454616
> Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
> Commit-Queue: Jochen Eisinger <jochen@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#43765}

TBR=jochen@chromium.org,bmeurer@chromium.org,adamk@chromium.org,hablich@chromium.org,v8-reviews@googlegroups.com,v8-mips-ports@googlegroups.com,v8-ppc-ports@googlegroups.com,v8-x87-ports@googlegroups.com
# Not skipping CQ checks because original CL landed > 1 day ago.
BUG=v8:6084

Change-Id: I06f60d409eccb4ad3abad14f5c9782bfe9122aea
Reviewed-on: https://chromium-review.googlesource.com/458379Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44106}