This implements a sparse vector class for rangers cache and uses it bey 
default except when the CFG is very small, in qhich case the original 
full vectors are faster.  It works like a normal vector cache (in fact 
it inherits from it), but uses a sparse bitmap to determine whether a 
vector element is set or not.  This provide better performance for 
clearing the vector, as well as during initialization.

A new param is added for this transition "vrp_vector_threshold" which 
defaults to 250.  Anything function with fewer than 250 basic blocks 
will use the simple vectors.  Various timing runs have indicated this is 
about the sweet spot where using the sparse bitmap overtakes the time 
required to clear the vector initially. Should we make ranger live 
across functions in the future, we'll probably want to lower this value 
again as clearing is significantly cheaper.

This patch also rename the "evrp_*" params to "vrp_*" as there really is 
not a serperate EVRP pass any more, its all one vrp pass.   Eventually 
we'll probably want to change it to vrp1, vrp2 and vrp3 rather than 
evrp, vrp1  and vrp2.    But thats a task for later, perhaps when we 
reconsider pass orderings..

Bootstrapped on x86_64-pc-linux-gnu with no regressions.  Pushed.

Andrew