[docs patch]: Remove obsolete section in tree-ssa.texi, add V_MUST_DEF iterator

[Diego Novillo]

Daniel Berlin wrote:

> -@defmac NUM_USES (@var{ops})
> -Return the number of USE operands in array @var{ops}.
> -@end defmac
> -
Hmm, NUM_* are useful regardless of whether we want to force 
optimizations to use iterators.  Useful in passes that want to ignore 
statements with virtual operands, for instance.  For 4.1, those macros 
ought to disappear.

Other than that, I agree with the other changes.


Diego.