Just two functions instead of partially specialized structs. Also, no
need to compute the return type(s), just use {auto}.

R=tebbi@chromium.org

Bug: v8:9396
Change-Id: I840af52c3caac622aded8bd7656a5437abb2c8ef
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1801845
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63756}