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.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/1237813002

Cr-Commit-Position: refs/heads/master@{#29681}