Register currently has several methods twice: Once for regular
{Register} objects, once for constexpr registers or register codes. It
was implemented this way so that the non-constexpr code can include
DCHECKs.
With C++14, we can add these DCHECKs also in the constexpr methods. Thus
the redundant implementation can be skipped.

R=tebbi@chromium.org

Bug: v8:9810
Change-Id: Ifc6253d4cd04b19be9bca47495186849118ad6b8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1910958Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64953}