This will be used to compute the base pointer of the new unified
representation for both on-heap and external typed arrays. The idea is
that either the external or the tagged pointer is 0 (although in
practice, if the tagged pointer is non-0, the external pointer will
contain the offset from the start of the on-heap typed array to the data
in the on-heap typed array).

The HAdd is marked as depending on new-space promotion, as the tagged
pointer might move during GC, and so the result of the addition needs to
be recomputed.

BUG=v8:3996
R=jarin@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1244693002

Cr-Commit-Position: refs/heads/master@{#29760}