[Polly] Drop overapproximation of kernel arguments

[philip.pfaffe]

Summary

When generating the GPU kernel function, we used to overapproximate
the function arguments by pre-populating them from the IDtoValue Map, and then
removing things we know statically or things that are computed in different
ways. However, right after this, we walk the entire scop and pick up SCEVs and
their unknown Values, and add them to the kernel arguments. Because of this, we
can just drop the overapproximation in the first place.

lib/CodeGen/PPCGCodeGeneration.cpp

 std::tuple<SetVector<Value *>, SetVector<Function *>, SetVector<const Loop *>>
 GPUNodeBuilder::getReferencesInKernel(ppcg_kernel *Kernel) {
   SetVector<Value *> SubtreeValues;
   SetVector<const SCEV *> SCEVs;
   SetVector<const Loop *> Loops;
   SubtreeReferences References = {
       LI, SE, S, ValueMap, SubtreeValues, SCEVs, getBlockGenerator()};
 
-  for (const auto &I : IDToValue)
-    SubtreeValues.insert(I.second);
-
   // NOTE: this is populated in IslNodeBuilder::addParameters
   // See [Code generation of induction variables of loops outside Scops].
   for (const auto &I : OutsideLoopIterations)
     SubtreeValues.insert(cast<SCEVUnknown>(I.second)->getValue());
 
   isl_ast_node_foreach_descendant_top_down(
       Kernel->tree, collectReferencesInGPUStmt, &References);