The kInt64, kUint64, and kIntPtr type definitions made no sense.
This CL removes kIntPtr and fixes+renames k(I|Ui)nt64. It also adds
some DCHECKs to avoid similar bugs in the future.

Bug: chromium:1202609
Change-Id: Ibe2e9c7501c22959b850d2b1076e819455440450
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2851895Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74230}