This patch implements the suggestion that we have an alternative 
ssa-cache which does not zero memory, and instead uses a bitmap to track 
whether a value is currently set or not.  It roughly mimics what 
path_range_query was doing internally.

For sparsely used cases, expecially in large programs, this is more 
efficient.  I changed path_range_query to use this, and removed it old 
bitmap (and a hack or two around PHI calculations), and also utilized 
this is the assume_query class.

Performance wise, the patch doesn't affect VRP (since that still uses 
the original version).  Switching to the lazy version caused a slowdown 
of 2.5% across VRP.

There was a noticeable improvement elsewhere.,  across 230 GCC source 
files, threading ran over 12% faster!.  Overall compilation improved by 
0.3%  Not sure it makes much difference in compiler.i, but it shouldn't 
hurt.

bootstraps on x86_64-pc-linux-gnu with no regressions.   OK for trunk?  
or do you want to wait for the next release...

Andrew