The existing test did not take into account the implicit
dereference of &fixedArray and thus heap-escaped when it
was not necessary.

Also added a detailed test for this and related cases.

Fixes #12588

Change-Id: I951e9684a093082ccdca47710f69f4366bd6b3cf
Reviewed-on: https://go-review.googlesource.com/15130Reviewed-by: Dmitry Vyukov <dvyukov@google.com>