maybe_set_unchanging

[Richard Kenner]

    I don't remember the motivation at the moment, but one case that comes
    to mind is a loop where a variable declared const is initialized to a
    different value in each iteration.

Oh, because of concerns about being hoisted out of a loop?

This is interesting: does "stored once" mean stored once *statically* or
*dynamically*: we've never quite been clear about that.  I've interpreted
it in the former way, but perhaps it really means the latter.