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

Drive-by: Reuse helper function for stack slot loads on ia32 and x64.

R=clemensb@chromium.org

Bug: v8:10576
Change-Id: Iea5ad2f0982c443cf2297227e9a2367cbb14581f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2264099Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68535}