[mlir][linalg] Tune getTiedIndexingMap method (NFC).

[gysit]

Summary

Optimize the performance by using the range directly.

[benvanik]

awesome!

mlir/include/mlir/Dialect/Linalg/IR/LinalgInterfaces.td

InterfaceMethod<
         Return the input or output indexing map for `opOperand`.
       }],
       /*retTy=*/"AffineMap",
       /*methodName=*/"getTiedIndexingMap",
       /*args=*/(ins "OpOperand*":$opOperand),
       /*methodBody=*/"",
       /*defaultImplementation=*/[{
         assert(opOperand->getOwner() == this->getOperation());
-        return getIndexingMaps()[opOperand->getOperandNumber()];
+        auto indexingMaps =
+          $_op.indexing_maps().template getAsValueRange<AffineMapAttr>();
+        return *(indexingMaps.begin() + opOperand->getOperandNumber());
       }]
     >,
     InterfaceMethod<
       /*desc=*/[{
         Return the result tied to `opOperand`.
       }],
       /*retTy=*/"OpResult",
       /*methodName=*/"getTiedOpResult",