[mlir][Linalg] Add interface methods to get lhs and rhs of contraction op.

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

def LinalgContractionOpInterface : OpInterface<"ContractionOpInterface"> {
    `vector.contract`. All other ops are handled in a generic fashion.
    In the future, we may wish to allow more input arguments and elementwise and
    constant operations that do not involve the reduction dimension(s).
   }];
   let cppNamespace = "::mlir::linalg";
   let verify = [{ return detail::verifyContractionInterface($_op); }];
   let methods = [
     InterfaceMethod<
+    /*desc=*/"Returns the left-hand side operand.",
+    /*retTy=*/"Value",
+    /*methodName=*/"lhs",
+    /*args=*/(ins),
+    /*methodBody=*/[{
+      return $_op.getOperation()->getOperand(0);
+    }]>,
+    InterfaceMethod<
+    /*desc=*/"Returns the right-hand side operand.",
+    /*retTy=*/"Value",
+    /*methodName=*/"rhs",
+    /*args=*/(ins),
+    /*methodBody=*/[{
+      return $_op.getOperation()->getOperand(1);
+    }]>,
+    InterfaceMethod<
     /*desc=*/[{
       Returns whether the given op has indexing maps that correspond to a
       row-major matmul operation.
     }],
     /*retTy=*/"bool",
     /*methodName=*/"isRowMajorMatmul",
     /*args=*/(ins),
     /*methodBody=*/[{