Help with getting started with adding IPA-related attribute

[Daniel Santos]

> 1.) decides rather or not to inline a function,
> 2.) decides rather or not to make a .constprop version of a function,
> 3.) a good pass (when all constant propagation is done) to search for 
> fn parameters and variables (marked with an attribute) that were not 
> constproped away, and finally
> 4.) what mechanism should I use for that search?  (iterate through the 
> tree to find them and then see if they have rtl?  I haven't worked in 
> this area yet.

I've got 3 & 4 worked out as well as most of 1 & 2.  I'm learning the 
CFG which is quite complex and fascinating.  It would appear that it is 
already performing some of the constant propagation that I wanted it to 
do (without the new attribute), and yet many constancy tests fail unless 
the function is "always_inline"ed from the caller "flatten".  I'll have 
to dig more to figure out why.

Daniel