The semantics of that method are not obvious (it returns an empty
MaybeHandle if the element is undefined, otherwise it assumes it is of
type T). Since there is only a single use of that method in the whole
code base, just having that logic there explicitly is much simpler.

Potential future uses of that method are probably also easier to
understand if they just to the checks explicitly.

Drive-by: Document semantics of {FixedArray::GetValueChecked}. Also for
this method it's quesionable whether it is useful overall.
Drive-by^2: Avoid else after return (see style guide).

R=mstarzinger@chromium.org

Bug: v8:9183
Change-Id: I88dcb0f080d728bbe55932f17cf4d7eb25ab5928
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1619761
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61686}