The existing {OwnedVector::New} value-initializes all elements, which
means zeroing them in case on integral types. In many cases though we
know that we will overwrite the content anyway, so the initialization is
redundant.
In the case of assembly buffers for wasm compilation, this zeroing
showed up with several percent of execution times for some benchmarks.

Hence this CL introduces a new {OwnedVector::NewForOverwrite} (along the
lines of {std::make_unique_for_overwrite}), which only
default-initializes the values (meaning no initialization for integral
values).

R=thibaudm@chromium.org

Bug: v8:10576
Change-Id: I8d2806088acebe8a264dea2c7ed74b0423671d4f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2237140
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68268}