[MLIR] Fix incorrect assertion in memref replacement utility

[bondhugula]

Summary

The assertion had an unchecked use of getDefiningOp. The condition
itself is stale and not needed. This is a trivial/obvious fix - the
buggy path wasn't exercised since the current passes/users of this
utility always passed the result of an op as an "extra index".

mlir/lib/Dialect/Affine/Utils/Utils.cpp

if (indexRemap &&
     remapOutputs.assign(remapOperands.begin(), remapOperands.end());
   }
 
   SmallVector<Value, 4> newMapOperands;
   newMapOperands.reserve(newMemRefRank);
 
   // Prepend 'extraIndices' in 'newMapOperands'.
   for (Value extraIndex : extraIndices) {
-    assert(extraIndex.getDefiningOp()->getNumResults() == 1 &&
-           "single result op's expected to generate these indices");
     assert((isValidDim(extraIndex) || isValidSymbol(extraIndex)) &&
            "invalid memory op index");
     newMapOperands.push_back(extraIndex);
   }
 
   // Append 'remapOutputs' to 'newMapOperands'.
   newMapOperands.append(remapOutputs.begin(), remapOutputs.end());