[tree-ssa]: Easy way to get default def of a variable?

[Daniel Berlin]

Sometimes, when doing injury insertions, we need to know the default 
definition of a variable.
IE the first version of the variable.

The SSA rename routines know what it is, since they create it.
However, outside of them, it appears there is no easy way to find it.
I currently have it walk back through all the definitions (skipping 
ones we've seen to handle cyclic phi's properly) till it finds the one 
whose SSA_NAME_DEF_STMT is an empty statement.

This works, but it's slow, and it *seems* there should be a table or 
something of the "first versions" of variables.

I believe this is what Open64 refers to as the "zero version", and it 
has such a table, but i may be wrong.

--Dan