[RewriteStatepointsForGC] All constant should have null base pointer

Currently we consider that each constant has itself as a base value. I.e "base(const) = const". 
This introduces couple of problems when we are trying to avoid reporting constants in statepoint live sets:

1. When querying "base( phi(const1, const2) )" we will get "phi(const1, const2)" as a base pointer. Since 
   it's not a constant we will record it in a stack map. However on practice we don't want this to happen
   (constant are never relocated).
2. base( phi(const, gc ptr) ) = phi( const, base(gc ptr) ). This particular case imposes challenge on our 
   runtime - we don't expect to see constant base pointers other than null. This problems can be avoided 
   by treating all constant as if they were derived from null pointer base. I.e in a first case we will 
   not include constant pointer in a stack map at all. In a second case we will get "phi(null, base(gc ptr))" 
   as a base pointer which is a lot more convenient.

Differential Revision: http://reviews.llvm.org/D20584

llvm-svn: 270993

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