original commit message:

    This changes the calling convention of the CallConstructStub to take
    the original constructor (i.e. new.target in JS-speak) in a register
    instead of magically via the operand stack. For optimizing compilers
    the operand stack doesn't exist, hence cannot be peeked into.

BUG=

Review URL: https://codereview.chromium.org/1235273003

Cr-Commit-Position: refs/heads/master@{#29695}