This adds some tweaks to the ssa-range cache.

1)   Adds a new merge_range which works like set_range, except if there 
is already a value, the two values are merged via intersection and 
stored.   THis avpoids having to check if there is a value, load it, 
intersect it then store that in the client. There is one usage pattern 
(but more to come) in the code base.. change to use it.

2)  The range_of_expr() method in ssa_cache does not set the stmt to a 
default of NULL.  Correct that oversight.

3)  the method empty_p() is added to the ssa_lazy_cache class so we can 
detect if the lazy cache has any active elements in it or not.

Bootstrapped on 86_64-pc-linux-gnu with no regressions.   Pushed.

Andrew