Pack (mostly) stack TNs according to lexical scope information

Pack (mostly) stack TNs according to lexical scope information

 Packing TNs from shallow scopes before more deeply nested one
 is a perfect elimination order when the live ranges span the
 full scope (the interference graph is a comparability graph).
 Use that as a heuristic, and do that for TNs that are known
 to have such simple live ranges before the rest: this ensures
 that bad TNs don't mess everything up.

 The result is much tighter stack allocation (most of the effect
 comes from initialising stack frames at a smaller size, and growing
 less aggressively), and fewer long-lived stray references.

 Incidentally: fix catch block packing on win32, solving lp#1072739