Test for the HeapConstant reduction.

Move heap_constants to a scope where it can be reused by other tests.

Bug: v8:7703
Change-Id: I1da1dd7ad65670980867aa5319b96cc9c701c5a2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1876064
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64648}