Pay close attention to the RHS type alignment when rendering an
assignment op as it may differ from the LHS pointer one.

This problem was noticed when debugging a CI failure in #6648: due to
sheer luck the misalignment caused a segfault on macos that was also
reproduced locally.

I tried to write a small test case but it turned out to be a daunting
task as I couldn't manage to trigger the problem consistently (and stop
the optimizer from simplifying everything). Patches welcome.