Revert r314923: "Recommit : Use the basic cost if a GEP is not used as addressing mode"

Significantly reduces performancei (~30%) of gipfeli
(https://github.com/google/gipfeli)

I have not yet managed to reproduce this regression with the open-source
version of the benchmark on github, but will work with others to get a
reproducer to you later today.

llvm-svn: 315680
diff --git a/llvm/lib/Transforms/Scalar/NaryReassociate.cpp b/llvm/lib/Transforms/Scalar/NaryReassociate.cpp
index 3d5a513..d0bfe36 100644
--- a/llvm/lib/Transforms/Scalar/NaryReassociate.cpp
+++ b/llvm/lib/Transforms/Scalar/NaryReassociate.cpp
@@ -264,7 +264,7 @@
   SmallVector<const Value*, 4> Indices;
   for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I)
     Indices.push_back(*I);
-  return TTI->getGEPCost(cast<GEPOperator>(GEP),
+  return TTI->getGEPCost(GEP->getSourceElementType(), GEP->getPointerOperand(),
                          Indices) == TargetTransformInfo::TCC_Free;
 }