This passes the new.target value in a register instead of through a
side-channel via the construct stub. Note that this marks the last
consumer of said side-channel and the special slot in the construct
stub frame can be removed as a follow-up.

R=bmeurer@chromium.org,yangguo@chromium.org
TEST=mjsunit/es6/regress/regress-new-target-context

Review URL: https://codereview.chromium.org/1492793002

Cr-Commit-Position: refs/heads/master@{#32548}