[PATCH 4/6] gimple-range-cache

[Andrew MacLeod]

These are the various caches used by the ranger.

- non-null-ref :  Tracks non-null pointer dereferences in blocks so we 
can properly calculate non-null pointer ranges
- on entry cache : Block ranges tracks the calucalted values sof 
ssa-names on entry to each basic block.
- global cache:  Stores whats globally known. Also used to determine if 
a stmt has been evaluated already.

And finally, the "Ranger Cache" which combines all those caches together.

It does far more than that tho.

It links them with a gori-computes component so that when it is 
calculating an outgoing range, it can "seed" the value with whatever has 
already been computed.  ie, if a_4 is used within a basic block, it's 
value  can be seeded with whatever the range-on-entry cache says a_4 is 
for that block.

Furthermore, the ranger cache also takes responsible for populating the 
on-entry cache.  When the ranger asks the cache for the range of a_4 on 
entry to BB9, the ranger cache is responsible for either retrieving the 
stored value, or computing it the first time.

The first time computation simply looks back to the definition (or 
previously populated entries) and applies any intervening GORI edge 
calculations between the definition and this block. and propagates those 
values along the edges until the cache is filled.  Note no new range 
calculations are requested, its all based on what already known between 
all the caches and what GORI can do with those values.

Its the rangers responsibility to ensure before asking for the cache to 
be filled that any global ranges have been calculated.

This component is also where the iterative update engine is located, and 
it purely based on injecting a new range into the on-entry cache, 
checking if propagating that range to successor blocks changes anything, 
and if so propagating those changes.   The update engine is currently 
only used within the cache fill for handling back edges and certain 
"poor" values that were used.

The API for this component is again pretty simple.  It exposes the 
caches it maintains, as well as 2 entry points.

  ssa_range_in_bb  :   which is a virtual function used by the gori 
computes component to access the on-entry or global cache for an ssa-name
  block_range :  This is the entry point the ranger uses to ask for the 
on entry range of a name which may trigger a cache fill if needed.
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