Port 440a42b7

Original commit message:
    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=mstarzinger@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
TEST=mjsunit/es6/regress/regress-new-target-context
BUG=

Review URL: https://codereview.chromium.org/1494003004

Cr-Commit-Position: refs/heads/master@{#32585}