[MLIR][linalg] Preserve location during elementwise fusion

[GMNGeoffrey]

Summary

This otherwise loses a lot of debugging info and results in a painful
debugging experience.

mlir/lib/Dialect/Linalg/Transforms/ElementwiseOpFusion.cpp

for (OpOperand *opOperand : genericOp.getInputOperands()) {
           !controlFn(resultValue, *opOperand))
         continue;
 
       // The operands and the indexing_maps of the fused operation the same as
       // the operands and indexing_maps of the generic operations with the
       // values at the constant index dropped.
       SmallVector<AffineMap> fusedIndexMaps;
       SmallVector<Value> fusedOperands;
+      SmallVector<Location> fusedLocs{genericOp.getLoc()};
       fusedIndexMaps.reserve(genericOp.getNumInputsAndOutputs());
       fusedOperands.reserve(genericOp.getNumInputs());
+      fusedLocs.reserve(fusedLocs.size() + genericOp.getNumInputs());
       for (OpOperand *inputOperand : genericOp.getInputOperands()) {
         if (inputOperand == opOperand)
           continue;
+        Value inputValue = inputOperand->get();
         fusedIndexMaps.push_back(genericOp.getTiedIndexingMap(inputOperand));
-        fusedOperands.push_back(inputOperand->get());
+        fusedOperands.push_back(inputValue);
+        fusedLocs.push_back(inputValue.getLoc());
       }
       for (OpOperand *outputOperand : genericOp.getOutputOperands())
         fusedIndexMaps.push_back(genericOp.getTiedIndexingMap(outputOperand));
 
       // Check if the operation shapes to loops map is computable.
       if (!inversePermutation(concatAffineMaps(fusedIndexMaps))) {
         return rewriter.notifyMatchFailure(
             genericOp, "fused op loop bound computation failed");
       }
 
       // Create a constant scalar value from the splat constant.
       Value scalarConstant = rewriter.create<ConstantOp>(
           def->getLoc(), constantAttr, constantAttr.getType());
 
       SmallVector<Value> outputOperands = genericOp.getOutputOperands();
       auto fusedOp = rewriter.create<GenericOp>(
-          rewriter.getUnknownLoc(), genericOp->getResultTypes(),
+          rewriter.getFusedLoc(fusedLocs), genericOp->getResultTypes(),
           /*inputs=*/fusedOperands,
           /*outputs=*/outputOperands,
           rewriter.getAffineMapArrayAttr(fusedIndexMaps),
           genericOp.iterator_types(),
           /*doc=*/nullptr,
           /*library_call=*/nullptr);
 
       // Map the block argument corresponding to the replaced argument with the