This operator doesn't generate any actual code, but teaches the register
allocator that a certain computed pointer value is tagged. This is
required to safely implement InnerAllocate (and we also use this for
Allocate to be sure that we don't suddenly leak a dangling pointer into
the heap somewhere).

R=epertoso@chromium.org
BUG=v8:4939
LOG=n

Review URL: https://codereview.chromium.org/1905813003

Cr-Commit-Position: refs/heads/master@{#35700}