Revert "[ConstraintElim] Treat ConstantPointerNull as constant offset 0."

This reverts commit 2362813.

Revert as this is causing some test failures.

Author: fhahn

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -455,9 +455,6 @@ static Decomposition decompose(Value *V,
   if (Ty->isPointerTy() && !IsSigned) {
     if (auto *GEP = dyn_cast<GEPOperator>(V))
       return decomposeGEP(*GEP, Preconditions, IsSigned, DL);
-    if (isa<ConstantPointerNull>(V))
-      return int64_t(0);
-
     return V;
   }