This CL fixes a flaky mjsunit test, that exercises Array#reduce with
sealed arrays in TurboFan. The flake was caused by temporary objects,
whos maps didn't live long enough. The code object of the function
under test holds weakly onto this maps. With a low enough gc interval,
the maps, and thus the code object, get cleaned up before the
{assertOptimized} can execute.

The fix is simply to assign these temporary objects to variables.

Bug: v8:9374
Change-Id: I43da8ba6b0194872b176e27617d9ca7fbfe43ec2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1666989
Auto-Submit: Simon Zünd <szuend@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62269}