Port 2332ebd8

Original Commit Message:

    - Add a separate function to load return slots, instead of encoding this
    in the offset,
    - Add fast path for single return.

R=thibaudm@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: Ia302772478b58fd25ee53a18e6ee03ac4b2ea9f5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2267477Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#68546}