This is to help the transition to using CSP as the stack pointer.
It does not make JSSP aligned yet, just makes sure that we only modify
JSSP by a multiple of 16 bytes. To do this, we might need to add a padding
slot above the receiver.

BUG=

Review-Url: https://codereview.chromium.org/2938603002
Cr-Commit-Position: refs/heads/master@{#46030}