[tree-ssa] Eliminating unnecessary variables during out-of-ssa

[Daniel Berlin]

> Currently, I dont think we need anything that has a derefernced 
> variable
> as a PHI_RESULT.

For SSAPRE store movement and load movement, I need it.

In particular, i need the vdefs for load movement, and the uses/phi 
results for store movement.

Otherwise, i can't tell when a load is possibly killed without 
calculating the exact same info (in the case of vdefs), or the possible 
insertion points for the store (that's what the PHI's give us).

There are other optimizations i'm working on (loop related) that need 
this info, too.

Just because our basic opts don't need it, doesn't mean you should 
remove it.
Can't you just ignore it in out-of-ssa, rather than remove it 
altogether?

--Dan