This refactors the way the function-body-decoder maintains
its value stack: it now always calls the respective instruction's
interface function before updating its value stack (by dropping
input values and pushing results). The benefit is that interface
functions still see the original values in the decoder.

No change in observable behavior is intended.

Change-Id: I7618d11ff16675ef29ccb246371ac4fc85733955
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2732019
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73193}