The merge values of a block have to be initialized to their
static types, even if the actual values on the stack have
subtypes of the loop's static type.

Drive-by cleanup: drop some unneeded manual {TestModuleBuilder}
instantiations from existing tests. The test fixture provides
one anyway.

Bug: chromium:1234453
Change-Id: I39c7eae4b6a6d5124f29be92da5ee92ff7e20e57
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3068948Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76087}