Minor MC heap verification requires heap iterability. This however was
not directly ensured. Coincidentally, there was an unrelated call to
`Heap::Verify` that ensured `Heap::MakeHeapIterable` had been called,
so the precondition was met. This call was moved to an earlier point
by https://crrev.com/c/3497318 and, because of that, some combination
of flags now results in a crash.

This CL fixes the issue by directly ensuring heap iterability. It also
moves back the call to `Heap::Verify`, so that it takes place inside
the GC safepoint.

Bug: v8:12768
Change-Id: I2c66de0d0a735b84dd9435ff503e78bb3611ce55
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3569224Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79844}