[tree-ssa] Lazy updating of stmt operands

[law@redhat.com]

In message <1070818186.7334.30.camel@frodo.toronto.redhat.com>, Diego Novillo w
rites:
 >> > tree-dfa.c:compute_immediate_uses()
 >> 
 >> Which needs to pass through every single statement in the program. Not
 >> really terribly efficient.
 >> 
 >*shrug*, it's used by SSA-CCP.  Since def-use edges are only needed by
 >some passes, I don't think it would be worth our while trying to
 >maintain them in get_stmt_operands.
 >
 >But I'm ready to be convinced otherwise.  Say, with an implementation
 >comparing both approaches with timings over a reasonable code base (a
 >typical GCC bootstrap or one of the compile-time related PRs).
IMHO, when we have the need, then we'll expand on the immediate uses
interface -- either by having a way to incrementally update it, or 
rebuild it.

While it's nice to think about a world where you always have immediate uses,
you end up with FUD chains if you do that -- with ultimately a tangled nest
of pointers that is bloody impossible to work with in the real world.

jeff