[NVPTX] Fixes -Wrange-loop-analysis warnings

This avoids new warnings due to D68912 adds -Wrange-loop-analysis to -Wall.

Also removed the top-level const as requested by Aaron Ballman in similar
patches.

Differential Revision: https://reviews.llvm.org/D71812

llvm/lib/Target/NVPTX/NVPTXISelDAGToDAG.cpp

@@ -612,7 +612,7 @@ bool NVPTXDAGToDAGISel::tryEXTRACT_VECTOR_ELEMENT(SDNode *N) {
 
   // Find and record all uses of this vector that extract element 0 or 1.
   SmallVector<SDNode *, 4> E0, E1;
-  for (const auto &U : Vector.getNode()->uses()) {
+  for (auto U : Vector.getNode()->uses()) {
     if (U->getOpcode() != ISD::EXTRACT_VECTOR_ELT)
       continue;
     if (U->getOperand(0) != Vector)