Refactor to use iterVar

HalideIR

include/tvm/domain.h

include/tvm/expr.h

include/tvm/ir.h

include/tvm/operation.h

include/tvm/split.h

include/tvm/tensor.h

python/tvm/_ctypes/_api.py

python/tvm/collections.py

python/tvm/expr.py

python/tvm/function.py

python/tvm/tensor.py

src/c_api/c_api_function.cc

src/c_api/c_api_lang.cc

src/c_api/c_api_registry.h

src/lang/domain.cc

src/lang/expr.cc

src/lang/ir.cc

src/lang/tensor.cc

src/pass/ir_mutator.cc